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.cpp b/services/surfaceflinger/DisplayDevice.cpp
index fd09ae4..76bbe2c 100644
--- a/services/surfaceflinger/DisplayDevice.cpp
+++ b/services/surfaceflinger/DisplayDevice.cpp
@@ -483,7 +483,7 @@
     std::scoped_lock lock(mActiveModeLock);
     if (mDesiredActiveModeChanged) {
         // If a mode change is pending, just cache the latest request in mDesiredActiveMode
-        const Scheduler::ModeEvent prevConfig = mDesiredActiveMode.event;
+        const auto prevConfig = mDesiredActiveMode.event;
         mDesiredActiveMode = info;
         mDesiredActiveMode.event = mDesiredActiveMode.event | prevConfig;
         return false;
@@ -508,7 +508,7 @@
 
 void DisplayDevice::clearDesiredActiveModeState() {
     std::scoped_lock lock(mActiveModeLock);
-    mDesiredActiveMode.event = Scheduler::ModeEvent::None;
+    mDesiredActiveMode.event = scheduler::DisplayModeEvent::None;
     mDesiredActiveModeChanged = false;
 }