AVRCP: Fix crash when app change during shutdown

If you turned off Bluetooth and then immediately changed the app that
media keys went to, a NPE could be generated.

Test: shutdown bt then quick-switch to telecom immediately
Change-Id: Ifbca7446d94aa411b642b42131f603c8f9d1c31e
Bug: 37865933
(cherry picked from commit 7c7cc128e93e7e1db8a5ed7f69583ddc6d99493e)
1 file changed