A2DP: Do the connection task in the stack main thread

There is a profile queue to handle connection tasks sequentially, and
invokes the connect API in JNI thread while the previous task finishes.
However, since BtifAvStateMachine is running in the stack main thread,
it brings a race condition about the BtifAVPeer object between the JNI
and stack main threads. This change does A2DP connection task in the
main thread, so has no such race conditions.

Bug: 152595833
Test: make sure the opening event is in main thread manually
Change-Id: Ib40dc7c5fa3e839a894d6f7947ac8266eacd2022