Move Voicemail notification settings into util.

This utility localizes related functions/constants in a class,
in preparation for adding more functionality to make both
voicemail ringtone and vibration settings MSIM-capable, and also
handle associated necessary migrations.

Besides fixing voicemail notifications (before it did not update when
the value is changed), there should be no change in functionality.

+ Add utility for settings related to voicemail notifications.
+ Handle get/set ringtone uri in new util.
+ Handle get/set vibrate in new util.
+ Make keys for shared preferences independent of preference names.
There was an implicit relationship between these before. I think
it is safer if the two are not correlated, and the shared preferences
key needs to be changed shortly to consider subscription id anyways.
+ Handle saving ringtone uri in VoicemailRingtonePreference. For now
this has no change in behavior, but in a subsequent CL we will save
the ringtone by subscription id here.
- Don't use default values on VoicemailRingtonePreference; default
values use the preference's key as the shared preference key, but we
will need to use subscription ids for our keys.

Bug: 18232725
Change-Id: Ifb52f47073418c76ad4bcf86c57c0ed75d05d0ca
6 files changed