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
2 files changed