Prevent Null Pointer Exception caused by race condition in AVRCP init

The object |mMPList| could be null if AVRCP started handling messages before
the phone was unlocked. Now we always initialize the Media Player List.

Test: Manual testing
Bug: 32060313
Change-Id: I66cba51159586da8abe51297dba94a3e1dc1b067
1 file changed