SF: Add VsyncSchedule skeleton

Pull the Scheduler::VsyncSchedule struct and related code into its own
file, as it will be extended with more per-display state, e.g. reactor
registrations, resync state machine, etc.

Add <scheduler/Features.h> for feature flags. Move Scheduler into its
namespace.

Bug: 185535769
Test: libsurfaceflinger_unittest
Change-Id: I6e10893632c5abf40380df924791d1fcc27c3cc2
diff --git a/services/surfaceflinger/tests/unittests/CompositionTest.cpp b/services/surfaceflinger/tests/unittests/CompositionTest.cpp
index 8d2c078..eb5f31e 100644
--- a/services/surfaceflinger/tests/unittests/CompositionTest.cpp
+++ b/services/surfaceflinger/tests/unittests/CompositionTest.cpp
@@ -138,7 +138,7 @@
                 .WillRepeatedly(Return(FakeHwcDisplayInjector::DEFAULT_VSYNC_PERIOD));
         EXPECT_CALL(*vsyncTracker, nextAnticipatedVSyncTimeFrom(_)).WillRepeatedly(Return(0));
 
-        constexpr ISchedulerCallback* kCallback = nullptr;
+        constexpr scheduler::ISchedulerCallback* kCallback = nullptr;
         constexpr bool kHasMultipleConfigs = true;
         mFlinger.setupScheduler(std::move(vsyncController), std::move(vsyncTracker),
                                 std::move(eventThread), std::move(sfEventThread), kCallback,