Always index RingerModeAffected slices

When indexing slices, SliceDataConverter makes an exception for
RingerModeAffected slices. They can be !isAvailable() but still be
added to the list of slices.

Bug: 259084354

Test: Manual. Enable systemui DeviceConfig flag
volume_separate_notification. Open volume dialog.
Verify that ring & notification slice is split into ring slice and
notification slice.

Change-Id: Iaab30154025cd082b388c412da6ecd573b8d6fcf