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
diff --git a/services/surfaceflinger/Layer.h b/services/surfaceflinger/Layer.h
index 78a8427..c070539 100644
--- a/services/surfaceflinger/Layer.h
+++ b/services/surfaceflinger/Layer.h
@@ -465,6 +465,9 @@
     // Temporary - Used only for LEGACY camera mode.
     uint32_t getProducerStickyTransform() const;
 
+    // Loads the corresponding system property once per process
+    static bool latchUnsignaledBuffers();
+
     // -----------------------------------------------------------------------
 
     class SyncPoint