Separate Multi-part SMS setting from size

This does the foll.:

- Separates the toggle from the size (makes more sense)
- Defaults to true, so users find no diff in behavior
- Default convert value for MMS to 7
  (I would suggest that the sprint devices set it via overlay to -1)
- Generalize numberpicker dialog message

Change-Id: I5b6fb33c3cc56a4078d0170a080ccee840afdbf1