btif_a2dp_source: Inline the call to btif_a2dp_setup_codec

btif_a2dp_setup_codec is only called from btif_a2dp_start_session,
and scheduules a task on the a2dp_source_worker thread to be run
right before btif_a2dp_start_session_delayed

As such the call can be safely inlined in
btif_a2dp_start_session_delayed without chaning the behavior

Bug: 365022887
Test: m com.android.btservices
Test: manual streaming test
Flag: EXEMPT, no logical change
Change-Id: Iea312630471d456655e7372d954a2a014f609e4c