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.cpp b/services/surfaceflinger/Scheduler/EventThread.cpp
index 57661f1..74665a7 100644
--- a/services/surfaceflinger/Scheduler/EventThread.cpp
+++ b/services/surfaceflinger/Scheduler/EventThread.cpp
@@ -525,7 +525,7 @@
 
 bool EventThread::shouldConsumeEvent(const DisplayEventReceiver::Event& event,
                                      const sp<EventThreadConnection>& connection) const {
-    const auto throttleVsync = [&] {
+    const auto throttleVsync = [&]() REQUIRES(mMutex) {
         const auto& vsyncData = event.vsync.vsyncData;
         if (connection->frameRate.isValid()) {
             return !mVsyncSchedule->getTracker()