Bluetooth: Always scan while on pairing or DevicePicker page

* Modified DeviceListPreferenceFragment to have enable/disable scanning
  methods. In ENABLE state, each onScanningStateChanged(false) will
  restart another round of scanning
* Subclasses of DeviceListPreferenceFragment should call enable/disable
  scanning when scanning is needed for long period of time
* Currently, BluetoothPairingDetail and DevicePickerFragment call
  enableScanning() when Bluetooth is turned ON and call disableScanning
  when some device is picked in their lists
* Both BluetoothPairingDetail and DevicePickerFragment will re-enable
  scanning if pairing failed for selected device
* Added associated unit tests as well

Bug: 32172815
Test: make, pair Bluetooth device, send file over Bluetooth Opp
Change-Id: I99325e06aadd7b00e7a7ba6d6c282a6831859d8b