Call mutate on notification shade drawable before editing.

All drawables, by default, are shared in memory thus this would affect other
unintended elements to also have their alpha set.

Bug: 134407440
Test: Manual
Change-Id: Ieb26146b67c0cb7d2bfd3213993238be2da68c23

packages/CarSystemUI/src/com/android/systemui/statusbar/car/CarStatusBar.java

@@ -1061,9 +1061,10 @@ public class CarStatusBar extends StatusBar implements
             // shade is visible to the user. When the notification shade is completely open then
             // alpha value will be 1.
             float alpha = (float) height / mNotificationView.getHeight();
-            Drawable background = mNotificationView.getBackground();
+            Drawable background = mNotificationView.getBackground().mutate();
 
             background.setAlpha((int) (alpha * 255));
+            mNotificationView.setBackground(background);
         }
     }