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/DisplayDevice.h b/services/surfaceflinger/DisplayDevice.h
index 4b9718f..324145e 100644
--- a/services/surfaceflinger/DisplayDevice.h
+++ b/services/surfaceflinger/DisplayDevice.h
@@ -190,7 +190,7 @@
 
     struct ActiveModeInfo {
         DisplayModePtr mode;
-        scheduler::RefreshRateConfigEvent event = scheduler::RefreshRateConfigEvent::None;
+        scheduler::DisplayModeEvent event = scheduler::DisplayModeEvent::None;
 
         bool operator!=(const ActiveModeInfo& other) const {
             return mode != other.mode || event != other.event;