btif_av: Clear the pending start flag before restarting the audio session

Bug: 370333565
Bug: 378524655
Test: m com.android.btservices
Flag: com.android.bluetooth.flags.a2dp_clear_pending_start_on_session_restart
Change-Id: Ie7bc3a8525ed03fd58679cac6915f8c54f33b2a7