AudioService: strengthen A2DP device detection

If the intent BluetoothA2dp.ACTION_CONNECTION_STATE_CHANGED is broadcast
before AudioService is started, The A2DP device connection is never detected
by the audio framework resulting in audio not being routed to A2DP unless the device
is rebooted or the A2DP headset reconnected.

Make sure that A2DP device connection state is sampled when
boot completed event is received.

Issue 5665159

Change-Id: I04d82020afc00af28c5ea0bb9879ed55bcc9b6f3

Author: Eric Laurent