Add FlagManager SF class.

To handle Mendel experiment flags and system properties. Example method
is `demo_flag`.

Bug: 190769260
Test: make, atest libsurfaceflinger_unittest
Change-Id: If42630dc9c545d4117b616e862a21970193eda56
diff --git a/services/surfaceflinger/tests/unittests/FlagManagerTest.cpp b/services/surfaceflinger/tests/unittests/FlagManagerTest.cpp
new file mode 100644
index 0000000..0905cd1
--- /dev/null
+++ b/services/surfaceflinger/tests/unittests/FlagManagerTest.cpp
@@ -0,0 +1,143 @@
+/*
+ * Copyright 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <cstdint>
+#undef LOG_TAG
+#define LOG_TAG "FlagManagerTest"
+
+#include "FlagManager.h"
+
+#include <android-base/properties.h>
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+#include <log/log.h>
+#include <server_configurable_flags/get_flags.h>
+#include <optional>
+
+namespace android {
+
+using testing::Return;
+
+class MockFlagManager : public FlagManager {
+public:
+    MockFlagManager() = default;
+    ~MockFlagManager() = default;
+
+    MOCK_METHOD(std::string, getServerConfigurableFlag, (const std::string& experimentFlagName),
+                (const, override));
+};
+
+class FlagManagerTest : public testing::Test {
+public:
+    FlagManagerTest();
+    ~FlagManagerTest() override;
+    std::unique_ptr<MockFlagManager> mFlagManager;
+
+    template <typename T>
+    T getValue(const std::string& experimentFlagName, std::optional<T> systemPropertyOpt,
+               T defaultValue);
+};
+
+FlagManagerTest::FlagManagerTest() {
+    const ::testing::TestInfo* const test_info =
+            ::testing::UnitTest::GetInstance()->current_test_info();
+    ALOGD("**** Setting up for %s.%s\n", test_info->test_case_name(), test_info->name());
+    mFlagManager = std::make_unique<MockFlagManager>();
+}
+
+FlagManagerTest::~FlagManagerTest() {
+    const ::testing::TestInfo* const test_info =
+            ::testing::UnitTest::GetInstance()->current_test_info();
+    ALOGD("**** Tearing down after %s.%s\n", test_info->test_case_name(), test_info->name());
+}
+
+template <typename T>
+T FlagManagerTest::getValue(const std::string& experimentFlagName,
+                            std::optional<T> systemPropertyOpt, T defaultValue) {
+    return mFlagManager->getValue(experimentFlagName, systemPropertyOpt, defaultValue);
+}
+
+namespace {
+TEST_F(FlagManagerTest, getValue_bool_default) {
+    EXPECT_CALL(*mFlagManager, getServerConfigurableFlag).Times(1).WillOnce(Return(""));
+    const bool defaultValue = false;
+    std::optional<bool> systemPropertyValue = std::nullopt;
+    const bool result = FlagManagerTest::getValue("test_flag", systemPropertyValue, defaultValue);
+    ASSERT_EQ(result, defaultValue);
+}
+
+TEST_F(FlagManagerTest, getValue_bool_sysprop) {
+    const bool defaultValue = false;
+    std::optional<bool> systemPropertyValue = std::make_optional(true);
+    const bool result = FlagManagerTest::getValue("test_flag", systemPropertyValue, defaultValue);
+    ASSERT_EQ(result, true);
+}
+
+TEST_F(FlagManagerTest, getValue_bool_experiment) {
+    EXPECT_CALL(*mFlagManager, getServerConfigurableFlag).Times(1).WillOnce(Return("1"));
+    const bool defaultValue = false;
+    std::optional<bool> systemPropertyValue = std::nullopt;
+    const bool result = FlagManagerTest::getValue("test_flag", systemPropertyValue, defaultValue);
+    ASSERT_EQ(result, true);
+}
+
+TEST_F(FlagManagerTest, getValue_int32_default) {
+    EXPECT_CALL(*mFlagManager, getServerConfigurableFlag).Times(1).WillOnce(Return(""));
+    int32_t defaultValue = 30;
+    std::optional<int32_t> systemPropertyValue = std::nullopt;
+    int32_t result = FlagManagerTest::getValue("test_flag", systemPropertyValue, defaultValue);
+    ASSERT_EQ(result, defaultValue);
+}
+
+TEST_F(FlagManagerTest, getValue_int32_sysprop) {
+    int32_t defaultValue = 30;
+    std::optional<int32_t> systemPropertyValue = std::make_optional(10);
+    int32_t result = FlagManagerTest::getValue("test_flag", systemPropertyValue, defaultValue);
+    ASSERT_EQ(result, 10);
+}
+
+TEST_F(FlagManagerTest, getValue_int32_experiment) {
+    EXPECT_CALL(*mFlagManager, getServerConfigurableFlag).Times(1).WillOnce(Return("50"));
+    std::int32_t defaultValue = 30;
+    std::optional<std::int32_t> systemPropertyValue = std::nullopt;
+    std::int32_t result = FlagManagerTest::getValue("test_flag", systemPropertyValue, defaultValue);
+    ASSERT_EQ(result, 50);
+}
+
+TEST_F(FlagManagerTest, getValue_int64_default) {
+    EXPECT_CALL(*mFlagManager, getServerConfigurableFlag).Times(1).WillOnce(Return(""));
+    int64_t defaultValue = 30;
+    std::optional<int64_t> systemPropertyValue = std::nullopt;
+    int64_t result = getValue("flag_name", systemPropertyValue, defaultValue);
+    ASSERT_EQ(result, defaultValue);
+}
+
+TEST_F(FlagManagerTest, getValue_int64_sysprop) {
+    int64_t defaultValue = 30;
+    std::optional<int64_t> systemPropertyValue = std::make_optional(10);
+    int64_t result = getValue("flag_name", systemPropertyValue, defaultValue);
+    ASSERT_EQ(result, 10);
+}
+
+TEST_F(FlagManagerTest, getValue_int64_experiment) {
+    EXPECT_CALL(*mFlagManager, getServerConfigurableFlag).Times(1).WillOnce(Return("50"));
+    int64_t defaultValue = 30;
+    std::optional<int64_t> systemPropertyValue = std::nullopt;
+    int64_t result = getValue("flag_name", systemPropertyValue, defaultValue);
+    ASSERT_EQ(result, 50);
+}
+} // namespace
+} // namespace android