Replace NOT_SUPPORTED string constants with empty strings.

AVRCP was defining default state strings as NOT_SUPPORTED and hence the
UI would show NOT_SUPPORTED as the string for album/song info when no
player is selected on the phone. Replacing these strings with empty
helps UI make a more informed decision.

Bug: b/28120051

Change-Id: Ia8017dce784160d637a6653f70addb5258e7871e
(cherry picked from commit a525ff9f)