Remove a problematic empty update optimization.

This change removes an optimization from SurfaceFlinger that skipped
composition when it got window updates that had an empty dirty region.
This optimization caused problems because it would skip the hwcomposer
set call, which could leave the window's previous frame buffer bound to
an overlay plane.  When the application subsequently dequeued and tried
to lock its next buffer (which would be the buffer currently bound to
the overlay), the lock call would block until the next hwcomposer set
call (which may never happen).

Change-Id: I563b626a1d52c1f30eb82489eae0ceb4edc79936
Bug: 3138752