Give ongoing ringtone vibrations priority over new incoming vibrations.

This is similar to alarms, and currently behaves largely the same as ringtones
are often repeating or external (audio-coupled) - this just covers the
non-repeating case, in case an external component does the repeat.

Test: atest
Change-Id: I733d28a59c132d8c5644f8fcd14d11096fc21dc7