Protect committing child list behind visible regions changed

This can be further optimized but as a first pass checking for
visible regions will improve the buffer update path. In Layer.cpp,
we need to be sure updating Z-order sets the mVisibleRegionsChanged
flag directly rather than relying on doTransaction (as this now occurs
too late after being coalesced in to handlePageFlip).

Test: Existing tests pass. simpleperf
Bug: 186200583
Change-Id: I24be38594f13e774d20a2a32ad12676a4e3c3052