a2dp: Fix BT SCO Tx configuration during a2dp to sco transition

 -For voice call tx backend is configured with a2dp sampling rate
  leading to distorted audio during voice call.
 -Don't set tx backend sample rate for a2dp if abr is not enabled.

Change-Id: I7cdb0a5c98b550665cfaafb02cd9151579d7138c
1 file changed