DO NOT MERGE 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
(cherry picked from commit 0b7e45ebc5)

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

@@ -1072,9 +1072,10 @@ 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);
         }
     }