No need to fetch bitmap in NMM

Lockscreen wallpaper media artwork is unavailable since R. There is no
need to fetch the bitmap in NotificationMediaManager.Removing this part
of code eliminates the existence of ANR.
This CL removes other unnecessary code for notification media artwork,
which includes removing MediaArtworkProcessor.

Bug: 298315646
Bug: 273768759
Test: Manually checked the device and set a new wallpaper on lockscreen
with no issues.
Test: atest NotificationMediaManagerTest

Change-Id: I423548a70c4137ed51319f36abed584376b55790