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/Layer.cpp b/services/surfaceflinger/Layer.cpp
index 968a49d..b3ea94f 100644
--- a/services/surfaceflinger/Layer.cpp
+++ b/services/surfaceflinger/Layer.cpp
@@ -1293,8 +1293,8 @@
     mDrawingState.modified = true;
     setTransactionFlags(eTransactionNeeded);
 
-    mFlinger->mScheduler->recordLayerHistory(this, systemTime(),
-                                             LayerHistory::LayerUpdateType::SetFrameRate);
+    using LayerUpdateType = scheduler::LayerHistory::LayerUpdateType;
+    mFlinger->mScheduler->recordLayerHistory(this, systemTime(), LayerUpdateType::SetFrameRate);
 
     return true;
 }