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/DisplayTransactionTestHelpers.h b/services/surfaceflinger/tests/unittests/DisplayTransactionTestHelpers.h
index de5e9df..0a3437a 100644
--- a/services/surfaceflinger/tests/unittests/DisplayTransactionTestHelpers.h
+++ b/services/surfaceflinger/tests/unittests/DisplayTransactionTestHelpers.h
@@ -121,7 +121,7 @@
 
     mock::VsyncController* mVsyncController = new mock::VsyncController;
     mock::VSyncTracker* mVSyncTracker = new mock::VSyncTracker;
-    mock::SchedulerCallback mSchedulerCallback;
+    scheduler::mock::SchedulerCallback mSchedulerCallback;
     mock::EventThread* mEventThread = new mock::EventThread;
     mock::EventThread* mSFEventThread = new mock::EventThread;