Fix issue #1806217.

We were not clearing the background drawable if it had been set as a resource
reference.

policy/com/android/internal/policy/impl/PhoneWindow.java

@@ -954,7 +954,7 @@ public class PhoneWindow extends Window implements MenuBuilder.Callback {
 
     @Override
     public final void setBackgroundDrawable(Drawable drawable) {
-        if (drawable != mBackgroundDrawable) {
+        if (drawable != mBackgroundDrawable || mBackgroundResource != 0) {
             mBackgroundResource = 0;
             mBackgroundDrawable = drawable;
             if (mDecor != null) {