A2DP: Only unregister receiver if we registered

If A2DP isn't ever started, A2dpService.cleanup() would try to unregister
when we are not registered.

Bug: 63061079
Test: none
Change-Id: I7229625f19c07dddeb58d7ee0787c83f0164e3e8
(cherry picked from commit 6a5c11bb626571ec8049733743a5e65e7845a67d)
1 file changed