Defer surfaceflinger composition until data latch

If layer data is not yet ready, latchBuffer will return an empty region.
SurfaceFlinger::handlePageFlip will now check if any layer has a queued
frame which succeeded in latching data.
If no new frame is latched, handlePageFlip will return false.
That will defer refresh until the next vsync.
That reduces HWC and GPU load, improving framerates.
It prevents dEQP tests from timing out on some devices.

Test: dEQP-VK.wsi.android.incremental_present.scale_down.fifo.reference
Bug: 37439915
Change-Id: Id46c82f79953f3a147af77bb0e8e81a395c606b3