a2dp: Do not call btif_a2dp_on_started when BTA_AV_START_EVT is received as source and responder

btif_a2dp_on_started starts a2dp offload and sends back an acknowledgment
to the BT audio HAL, which is not at all expected in this case:

1. the BT audio HAL was not initiator of the request and
   invoking stream started can fail / cause errors in the HAL.
2. the stream is already being suspended (see code), sending the a2dp
   offload command here can fail if another link is already active.

Bug: 338959084
Bug: 341178856
Flag: com.android.bluetooth.flags.a2dp_ignore_started_when_responder
Test: m com.android.btservices
Test: Manual
Change-Id: I0675a21a7d227948c8ae2c671f9706c03e9b6517