Disable expensive rendering during static screen

Locking the GPU frequency to a high value causes a power regression, so
when there's no screen updates we should disable expensive rendering -
previously expensive rendering would always be enabled if the previous
frame required expensive rendering.

Bug: 188625644
Test: Swipe to recents and monitor gpu clock info
Change-Id: Ie41fa6286e851c0390295672b399ce29419ea90b
diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index f33df86..66cf683 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -329,6 +329,10 @@
     bool mDisableClientCompositionCache = false;
     void setInputWindowsFinished();
 
+    // Disables expensive rendering for all displays
+    // This is scheduled on the main thread
+    void disableExpensiveRendering();
+
 protected:
     // We're reference counted, never destroy SurfaceFlinger directly
     virtual ~SurfaceFlinger();