AVRCP Controller getCurrentCalls NPE

Check for null call list if media device and phone call device are
different.

Bug: 138256493
Test: Connect 2 phones, one HFP one A2DP, receive incomming call.
Change-Id: I5f5c85d65283add3790dfaa1f252c3a14127f822
(cherry picked from commit 26041cc8)

Merged-In: I5f5c85d65283add3790dfaa1f252c3a14127f822
Change-Id: I58538ae0faddec9a66a3a695f70a2c030681f603