Fix FloatingWidgetView post-animation fallback background handling

The background drawables should only be restored to the original view if
the values are non-null. The original view may have had a background drawable
be a drawable, just not one that's elligible for animation. In that
case the local reference is null. We should not restore that.

Bug: 169042867
Test: manual
Change-Id: I54eaac5632e3f4f25daee2c36d7609e8b2ffedaf
diff --git a/quickstep/src/com/android/quickstep/views/FloatingWidgetBackgroundView.java b/quickstep/src/com/android/quickstep/views/FloatingWidgetBackgroundView.java
index f74aa55..9ea2369 100644
--- a/quickstep/src/com/android/quickstep/views/FloatingWidgetBackgroundView.java
+++ b/quickstep/src/com/android/quickstep/views/FloatingWidgetBackgroundView.java
@@ -100,8 +100,8 @@
     /** Restores the drawables to the source view. */
     void finish() {
         if (isUninitialized()) return;
-        mSourceView.setForeground(mOriginalForeground);
-        mSourceView.setBackground(mOriginalBackground);
+        if (mOriginalForeground != null) mSourceView.setForeground(mOriginalForeground);
+        if (mOriginalBackground != null) mSourceView.setBackground(mOriginalBackground);
     }
 
     void recycle() {