PhonePolicy: Only auto connect to the last active A2DP device

* Only auto connect to the last device indicated by
  BluetoothA2dp.ACTION_ACTIVE_DEVICE_CHANGED intent
* Ignore the intent when EXTRA_DEVICE is null since we cannot tell
  if setting active device to null is due to user action or due to
  device disconnection. Consider revising this policy in future
  revisions of phone policy

Bug: 79995170
Test: PhonePolicyTest, connect and toggle Bluetooth
Change-Id: I0a0b34e69c6db206fcb6ca67e7edd6421a61f586
(cherry picked from commit f3bff5dc)