Fix benign integer overflow conditions for VSYNC add and removal.

There are integer overflow conditions that could occur on
vsyncsForLastFrame in VideoFrameScheduler::schedule upon a VSYNC
add or removal. Additional checks have been put in place to
remove the overflowable conditions.

Bug: 24980200
Change-Id: I7dfc25ae1d2f3e3b3382e990adb3c56518c64e8d
1 file changed