Work around VSync signal while screen is off.

Change-Id: I0a9637170ee658ff84a363e91c558892dce5cfaf

libs/rs/rsContext.cpp

@@ -263,6 +263,10 @@ void * Context::threadProc(void *vrsc) {
             rsc->timerSet(RS_TIMER_IDLE);
 
 #ifndef ANDROID_RS_SERIALIZE
+            if (!rsc->mRootScript.get() || !rsc->mHasSurface || rsc->mPaused) {
+                targetRate = 0;
+            }
+
             if (vsyncRate != targetRate) {
                 displayEvent.setVsyncRate(targetRate);
                 vsyncRate = targetRate;