EventThread: add annotations for mVsyncSchedule

Split out from patch set 1 of Ieb05fc3abcf4fc20f04ea5d3595da70155de2df5.
Now that mVsyncSchedule can be changed, document that is guarded by
mMutex.

Bug: 272943830
Test: make

Change-Id: Ie4e492fa667687b36f648b566e13552f96f0d559
diff --git a/services/surfaceflinger/Scheduler/EventThread.h b/services/surfaceflinger/Scheduler/EventThread.h
index 87e20a0..30869e9 100644
--- a/services/surfaceflinger/Scheduler/EventThread.h
+++ b/services/surfaceflinger/Scheduler/EventThread.h
@@ -205,7 +205,7 @@
     TracedOrdinal<int> mVsyncTracer;
     TracedOrdinal<std::chrono::nanoseconds> mWorkDuration GUARDED_BY(mMutex);
     std::chrono::nanoseconds mReadyDuration GUARDED_BY(mMutex);
-    std::shared_ptr<scheduler::VsyncSchedule> mVsyncSchedule;
+    std::shared_ptr<scheduler::VsyncSchedule> mVsyncSchedule GUARDED_BY(mMutex);
     TimePoint mLastVsyncCallbackTime GUARDED_BY(mMutex) = TimePoint::now();
     scheduler::VSyncCallbackRegistration mVsyncRegistration GUARDED_BY(mMutex);
     frametimeline::TokenManager* const mTokenManager;