Add a h/w composer API to allow the HAL to trigger a redraw

This is useful in various situations, for instance if the HAL
wants to change its compositing strategy.

Bug: 4488017
Change-Id: I5afc14e5917c6db7926d7417d48677d5aba50caa
diff --git a/services/surfaceflinger/DisplayHardware/DisplayHardware.h b/services/surfaceflinger/DisplayHardware/DisplayHardware.h
index cdf89fd..40a6f1e 100644
--- a/services/surfaceflinger/DisplayHardware/DisplayHardware.h
+++ b/services/surfaceflinger/DisplayHardware/DisplayHardware.h
@@ -95,6 +95,7 @@
     void init(uint32_t displayIndex) __attribute__((noinline));
     void fini() __attribute__((noinline));
 
+    sp<SurfaceFlinger> mFlinger;
     EGLDisplay      mDisplay;
     EGLSurface      mSurface;
     EGLContext      mContext;