Call DecorView.drawableChanged() in PhoneWindow.setBackgroungDrawable

Not calling it result in empty buffer and stuttering during some
animation.

BUG: 158672949
Test: Manual test with ASOS app (see b/151910308) and Boost app
(b/158672949)

Change-Id: Ic5018829d7c7dde388bc928c1f5b3f773cc2f5e4
diff --git a/core/java/com/android/internal/policy/PhoneWindow.java b/core/java/com/android/internal/policy/PhoneWindow.java
index 046981c..d90a022 100644
--- a/core/java/com/android/internal/policy/PhoneWindow.java
+++ b/core/java/com/android/internal/policy/PhoneWindow.java
@@ -1510,11 +1510,13 @@
         if (drawable != mBackgroundDrawable) {
             mBackgroundDrawable = drawable;
             if (mDecor != null) {
+                mDecor.startChanging();
                 mDecor.setWindowBackground(drawable);
                 if (mBackgroundFallbackDrawable != null) {
                     mDecor.setBackgroundFallback(drawable != null ? null :
                             mBackgroundFallbackDrawable);
                 }
+                mDecor.finishChanging();
             }
         }
     }