Perform all codec-related init operations on the same thread

Renamed btif_a2dp_source_encoder_init_event() to
btif_a2dp_source_setup_codec_delayed() and moved the
peer parameter fetching from btif_a2dp_source_setup_codec()
to btif_a2dp_source_setup_codec_delayed()

Test: Manual - A2DP connect/disconnect; audio streaming
Change-Id: Ib29dd6b2bef3045be3727122e1f8608780921d98