AVRCP: fix crash on callback with no player

If MediaSessionService tells us there's no place for keys to go, then we
have no player.

Test: disable com.google.music, reboot
Change-Id: I2adf884f9db991975cd0ae72abf2238ce323c517
Fixes: 37681487
(cherry picked from commit 41527b2493a4ea59933c7cd4f8d1c761598509d8)
1 file changed