SF: Only latch buffers after fence signals

Changes SurfaceFlinger to only latch a buffer after its corresponding
acquire fence has signaled. This will enable us to move
SurfaceFlinger closer to vsync since there is no risk of fence waits
blocking composition.

Bug: 29413700
Change-Id: I26f4fd600c1611b8d736ec654d1f0f02cf69ae5f