[Pair hearing devices] Add "Saved devices" to show bonded but not connected hearing devices

* BaseHearingDevicePreferenceController will also be used in "Hearing devices", so extract to parent class first.

Bug: 237625815
Test: make RunSettingsRoboTests ROBOTEST_FILTER="(SavedHearingDeviceUpdaterTest|BaseBluetoothDevicePreferenceControllerTest|BluetoothDeviceUpdaterTest|AvailableMediaBluetoothDeviceUpdaterTest|ConnectedBluetoothDeviceUpdaterTest|SavedBluetoothDeviceUpdaterTest)"
Change-Id: I8a492866f48e3a664b9ff78bce5a4f082c0dc465