Change respond-via-sms strings on locale switch.

- No longer automatically store default responses in shared
preferences when performing initial migration. Only store if
previous values are present.
- No longer persist the preferences for these responses.
+ Save preference values when changed in RespondViaSmsSettings.
If the user has changed the value, their value will appear.
Otherwise, the response will be the default for that locale.

Bug: 19501543
Change-Id: Ib0f673ff0ac97b98a9934d8f0a81090ce8e590ea