SurfaceFlinger: expectedVsyncTimestamp can be 0

Scheduler::isVsyncValid can be called with expectedVsyncTimestamp as 0
if it is called from Layer::latchAndReleaseBuffer. If we send the vsync
timestamp as 0 to VSyncTracker, it may change the phase for the current
divider which will cause jank.

Test: run bouncyball and observe systraces
Change-Id: Ic0b0ea895497891a22badc9c67cea5c23698fc2c