Bluetooth: reset mConnectionState when adapter is OFF

* No device is connected when Bluetooth adapter is OFF
* BluetoothSummaryUpdater should reset its connection state tracker in
  order to display the correct summary message on ConnectedDevice
  preference
* Otherwise, "Connected to null" will be shown because no device is
  connected while BluetoothSummaryUpdater is still in CONNECTED state
* Removed unused imports from BluetoothSummaryUpdater
* Write additional unit test to verify the above behaviour
* Add additional logging when deviceName is null in CONNECTED state

Bug: 62492716
Test: Pair and connect to Bluetooth device, turning Bluetooth ON/OFF,
      unit tests
Change-Id: I30726636f5678d61d6052f5b8d211aa20f26f409