Latch and relase buffers for offscreen layers on main thread.

The current code was calling fakeVsync, which was latching and releasing
buffers on a binder thread when a new frame arrived. This was causing a
race with the latchBuffer on the main thread. This change calls
latchAndReleaseBuffer on the main thread for offscreen layers to ensure
we don't block dequeueBuffer but also don't race the other latchBuffer
call.

Fixes: 139118508
Test: Race was only reproducible in monkey tests
Change-Id: Ie7d87aa775c215de3fb2322b73849402596cc3fc