Reset notification shade background tints when theme changes.

These colors were not reset, which could result in colors from the old theme remaining after switching users. This demonstrably solves an issue easily found with the shelf being the wrong color on the lockscreen after switching users.  This is also the very likely to be a fix for the notifications retaining the wrong background color after exiting setup wizard, it's just much harder to repeat that one really often.

Bug: 227880269
Test: atest ActivatableNotificationViewTest
Change-Id: I522f4919c428272da73f2500d68d8245cd1344d8