Remove cover artwork from storage once it is no longer in use

This change removes images from storage once they are no longer in both
the browse tree or the current track's metadata. This keeps storage from
getting too large and only keeps images we have metadata for.

Tag: #compatibility
Bug: 153461730
Test: Build, Flash, interop test with AVRCP Cover Art devices, atest
BluetoothInstrumentationTests

Change-Id: I01c65ce0c60c4f81c408152f5f986e6818150dd1