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/Scheduler/LayerHistory.h b/services/surfaceflinger/Scheduler/LayerHistory.h
index 92236f5..8d56951 100644
--- a/services/surfaceflinger/Scheduler/LayerHistory.h
+++ b/services/surfaceflinger/Scheduler/LayerHistory.h
@@ -31,11 +31,9 @@
 namespace android {
 
 class Layer;
-class TestableScheduler;
 
 namespace scheduler {
 
-class LayerHistoryTest;
 class LayerInfo;
 
 class LayerHistory {
@@ -75,8 +73,8 @@
     std::string dump() const;
 
 private:
-    friend LayerHistoryTest;
-    friend TestableScheduler;
+    friend class LayerHistoryTest;
+    friend class TestableScheduler;
 
     using LayerPair = std::pair<Layer*, std::unique_ptr<LayerInfo>>;
     using LayerInfos = std::vector<LayerPair>;