Use broadcast receiver to update BT A2DP status

Audio routes observer in AudioService could have several seconds
delay. To mitigate the issue because of the delay, this CL uses
a BroadcastReceiver to get the Bluetooth device connection events.

Bug: 70175199
Test: manually and passed Media CTS
Change-Id: Ic1e9db1b8d5759070962011d7f2fc4f940ce8497