Wait on frame work fences when frames are dropped

bug:30895941

Prevents a race where frame work could interleave between frames,
causing SurfaceView position updates to be delivered out of order.

Change-Id: I01e4cc557b69dcf33e877a0e16c0d115ec95e4cc

libs/hwui/renderthread/DrawFrameTask.cpp

@@ -104,6 +104,9 @@ void DrawFrameTask::run() {
 
     if (CC_LIKELY(canDrawThisFrame)) {
         context->draw();
+    } else {
+        // wait on fences so tasks don't overlap next frame
+        context->waitOnFences();
     }
 
     if (!canUnblockUiThread) {