Free prefetched layers on TRIM_MEMORY

 Bug: 17208461

Change-Id: I831c10d29d5920274b90e11a67d6cd106972a058
diff --git a/libs/hwui/renderthread/CanvasContext.cpp b/libs/hwui/renderthread/CanvasContext.cpp
index ecfedf6..491a295 100644
--- a/libs/hwui/renderthread/CanvasContext.cpp
+++ b/libs/hwui/renderthread/CanvasContext.cpp
@@ -314,6 +314,7 @@
     stopDrawing();
     if (mEglManager.hasEglContext()) {
         requireGlContext();
+        freePrefetechedLayers();
         mRootRenderNode->destroyHardwareResources();
         Caches::getInstance().flush(Caches::kFlushMode_Layers);
     }