White media notifications are now dimmable

Certain notifications would stay fully white on the
lock screen, when they didn't provide an image.
This looked pretty bad and it made the colorization
slightly unclear.

Change-Id: I8369f5e41ffdf31b1672ae9282fccaaef2294a2e
Fixes: 69171439
Test: add media notification without image, observe normal dimming

packages/SystemUI/src/com/android/systemui/statusbar/notification/NotificationMediaTemplateViewWrapper.java

@@ -58,6 +58,6 @@ public class NotificationMediaTemplateViewWrapper extends NotificationTemplateVi
 
     @Override
     public boolean isDimmable() {
-        return false;
+        return getCustomBackgroundColor() == 0;
     }
 }