WallpaperManagerService: Get rid of removed UIDs that have set dim

If the UID changes while a dim was set it'd never be set back to 0
This was actually triggered for some users:

setting the dim with no UID via adb immediately reverted to whatever the app set
dumpsys wallpaper showed a UID under mUidToDimAmount
only settings the dim for that UID via adb allowed having 0 dim back again.

Change-Id: I3c2a90e60c4fd3e5339f0276a5f176ee9cd91c59
1 file changed