Persist ints and force input type to integer in NumberRangePreference.

@flerda
A little more work on NumberRangePreference. Two changes here:
1 - We shouldn't rely on setting the input type as integer-only in the XML definition. Do it programmatically instead..

2 - Persist and retrieve the values as ints instead of strings. At the moment, we aren't persisting the value (to a shared preference) so it won't have any effect, but we'll need it for other settings later. I'll add a getter and setter in a future commit since it'll help in the other preference work I'm doing now.