Close scan screen when pairing and remove device when unpairing.

Close the scan screen after successful pairing, and remove a
device from the list of paired devices after unpairing.

As part of the fix, BluetoothSettings was refactored into a parent
class, DeviceListPreferenceFragment, and three subclasses for each
variant type: BluetoothSettings, BluetoothFindNearby, and
DevicePickerFragment, replacing the checks against mScreenType with
custom logic in the child classes.

Bug: 3325848
Change-Id: If64fddc3ba5b4f1136451491c7d5a1139b696e47