PBAP: Add cleanUpServerSocket helper and remove unused code

* Remve broadcast receiver for BluetoothAdapter.ACTION_STATE_CHANGED.
When Bluetooth adapter status changed, lifecycle method will be called.
* Remove mAdapter != null check.
* Remove unused private fields.
* Remove MSG_SESSION_DISCONNECTED because it is replaced by
MSG_SERVERSESSION_CLOSE.
* Add helper method cleanUpServerSocket().
* Limit the scope of private fields.

Test: carkits with PBAP
Bug: 69417672
Change-Id: I8c144fa1c500db18c18e87cf20ec63c4223d2878