fix a bug that caused the off animation to not show sometimes

this happened when the overlays were in use, since the animation
is rendered in the FB and the FB is not used.

we now have a way to turn hwc off temporarily.

Change-Id: I3385f0c25bb9cc91948e7b26e7cd31ed18c36ace
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.h b/services/surfaceflinger/DisplayHardware/HWComposer.h
index 983898a..77c1a4b 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.h
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.h
@@ -50,6 +50,9 @@
     // Asks the HAL what it can do
     status_t prepare() const;
 
+    // disable hwc until next createWorkList
+    status_t disable();
+
     // commits the list
     status_t commit() const;