HWC2: Add properties to revert latching changes

Adds two properties:

debug.sf.latch_unsignaled - This causes SurfaceFlinger to latch
  buffers even if their fences haven't signaled
debug.sf.disable_backpressure - This causes SurfaceFlinger to skip
  propagating backpressure back to apps by not running when it
  detects that a prior composition missed hardware vsync

Bug: 30216498
Change-Id: Idd70f472ec567a4bc03352f30f201f434cf5d660