SF: Encapsulate frame targeting

Introduce FrameTargeter to isolate a display's per-frame metrics around
past/upcoming deadline targets. The Scheduler updates the FrameTargeter
on frame begin/end, whereas ICompositor (concretely SurfaceFlinger) has
read-only access via the FrameTarget interface.

For now, only instantiate the pacesetter's FrameTargeter.

The only functional change is that `earliestPresentTime` now takes into
account the case of targeting two VSYNCs ahead.

Bug: 241285475
Bug: 241285191
Test: Perfetto
Test: dumpsys SurfaceFlinger --scheduler
Test: atest libscheduler_test:FrameTargeterTest
Change-Id: Idf9f43b37f3479c94a478d154eaa46f43e0c6c9d
diff --git a/services/surfaceflinger/Scheduler/VsyncSchedule.h b/services/surfaceflinger/Scheduler/VsyncSchedule.h
index 763d058..9867b41 100644
--- a/services/surfaceflinger/Scheduler/VsyncSchedule.h
+++ b/services/surfaceflinger/Scheduler/VsyncSchedule.h
@@ -19,14 +19,17 @@
 #include <memory>
 #include <string>
 
-#include <ThreadContext.h>
 #include <android-base/thread_annotations.h>
 #include <ftl/enum.h>
 #include <ftl/optional.h>
-#include <scheduler/Features.h>
-#include <scheduler/Time.h>
 #include <ui/DisplayId.h>
 
+#include <scheduler/Features.h>
+#include <scheduler/IVsyncSource.h>
+#include <scheduler/Time.h>
+
+#include "ThreadContext.h"
+
 namespace android {
 class EventThreadTest;
 class VsyncScheduleTest;
@@ -49,13 +52,14 @@
 using VsyncTracker = VSyncTracker;
 
 // Schedule that synchronizes to hardware VSYNC of a physical display.
-class VsyncSchedule {
+class VsyncSchedule final : public IVsyncSource {
 public:
     VsyncSchedule(PhysicalDisplayId, FeatureFlags);
     ~VsyncSchedule();
 
-    Period period() const;
-    TimePoint vsyncDeadlineAfter(TimePoint) const;
+    // IVsyncSource overrides:
+    Period period() const override;
+    TimePoint vsyncDeadlineAfter(TimePoint) const override;
 
     // Inform the schedule that the period is changing and the schedule needs to recalibrate
     // itself. The schedule will end the period transition internally. This will