Make the dialpad silent in "vibrate only" mode (bug 1984905).

This is a followup to https://android-git.corp.google.com/g/5438 (see bug
1839556) which made the dialpad silent if the phone was in "silent mode".

But "vibrate only" mode has a separate AudioManager constant:
RINGER_MODE_VIBRATE.  I need to check for that too.