HearingAid: set playback_started to true when start audio command finish.

Bug: 205679750
Tag: #compatibility
Test: 1. Paring from setting and and streaming play
      2. HA battery remove and back reconnect
      3. out of range reconnect
      4. Paired from APP

Change-Id: I0a3e58b792bb12aa86f8ca9e1fe7a9d7bca58ec7