Remove volume_separate_notification flag

Replace it with true. This flag has been true for all UDC devices since
2022-02-17 so this is expected to have no user-visible effect, except
in cases where devices aren't getting server-side flag updates.

Bug: b/255363741

Test: atest VolumeDialogImplTest AudioServiceTest

Change-Id: I036dbe8611bf022c1e7c5878fd17ba5a61aca99d