Start the media encoder for a successful BTA_AV_START_EVT

When phone is A2DP source and receives BTA_AV_START_EVT, we need to
proceed to start the encoder of software or offload, and ack back to
audio HAL. When issue happened, the remote as the AVDTP_START initiator
triggered BTA_AV_START_EVT, and caused we did not start the encoder to
stream the audio. This change simplified the handler of BTA_AV_START_EVT
to always start the encoder for a successful event, and ack to audio HAL
if we are AVDTP initiator.

Bug: 126136429
Test: A2DP play and paused manually on Pixel 2 and Pixel 3
Change-Id: Ib857ea8b74384c6e82ac19ee6128b83f6b16d6fe