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.cpp b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
index 7d1bdf0..0ff1cce 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.cpp
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
@@ -108,8 +108,21 @@
 }
 
 status_t HWComposer::release() const {
-    int err = mHwc->set(mHwc, NULL, NULL, NULL);
-    return (status_t)err;
+    if (mHwc) {
+        int err = mHwc->set(mHwc, NULL, NULL, NULL);
+        return (status_t)err;
+    }
+    return NO_ERROR;
+}
+
+status_t HWComposer::disable() {
+    if (mHwc) {
+        free(mList);
+        mList = NULL;
+        int err = mHwc->prepare(mHwc, NULL);
+        return (status_t)err;
+    }
+    return NO_ERROR;
 }
 
 size_t HWComposer::getNumLayers() const {