more precise sms controls

### New SMS controls (in their own category now)

The normal "defaults" controls (changes based on Android version)


Use SMS with push unavailable
`[ Always | Only for secure conversation | Never ]`

Ask before falling back to SMS (checkbox)
`[ Yes | No ]` (checkbox)

### Open questions before PR is closed
* When SMS is not the default, what if any preferences would be disabled? Do we allow outgoing SMS when push is unavailable even when we're not the default SMS application?
* Do we only show non-push contacts in selection lists when the user has chosen "Always"?