Fix warnings for BufferHubServer_test in oc branch

Clang is complaining about signed and unsigned comparing in oc branch.
This CL will fix it.

Test: BufferHubServer_test (passed)
Bug: none
Change-Id: Iddb9b87c11975f4bfbb524570ff63aedeaa58659
diff --git a/services/bufferhub/tests/Android.bp b/services/bufferhub/tests/Android.bp
index e565374..8d29923 100644
--- a/services/bufferhub/tests/Android.bp
+++ b/services/bufferhub/tests/Android.bp
@@ -8,6 +8,8 @@
         "-DLOG_TAG=\"BufferHubServer_test\"",
         "-DTRACE=0",
         "-DATRACE_TAG=ATRACE_TAG_GRAPHICS",
+        "-Wall",
+        "-Werror",
     ],
     header_libs: [
         "libdvr_headers",
diff --git a/services/bufferhub/tests/BufferHubIdGenerator_test.cpp b/services/bufferhub/tests/BufferHubIdGenerator_test.cpp
index 4eddfe0..fe01013 100644
--- a/services/bufferhub/tests/BufferHubIdGenerator_test.cpp
+++ b/services/bufferhub/tests/BufferHubIdGenerator_test.cpp
@@ -27,7 +27,7 @@
     // resulting IDs should still keep incresing.
     const size_t kTestSize = 10U;
     uint32_t ids[kTestSize];
-    for (int i = 0; i < kTestSize; ++i) {
+    for (size_t i = 0UL; i < kTestSize; ++i) {
         ids[i] = mIdGenerator->getId();
         EXPECT_NE(ids[i], BufferHubIdGenerator::kInvalidId);
         if (i >= 1) {
diff --git a/services/bufferhub/tests/BufferNode_test.cpp b/services/bufferhub/tests/BufferNode_test.cpp
index dbf10e8..ccb1197 100644
--- a/services/bufferhub/tests/BufferNode_test.cpp
+++ b/services/bufferhub/tests/BufferNode_test.cpp
@@ -74,7 +74,7 @@
 
     for (int i = 0; i < BufferHubDefs::kMaxNumberOfClients; ++i) {
         new_client_state_mask = buffer_node->AddNewActiveClientsBitToMask();
-        EXPECT_NE(new_client_state_mask, 0);
+        EXPECT_NE(new_client_state_mask, 0U);
         EXPECT_FALSE(new_client_state_mask & current_mask);
         expected_mask = current_mask | new_client_state_mask;
         current_mask = buffer_node->GetActiveClientsBitMask();