Move default SMS and system import to "reminders"

Remove the system SMS import question at registration and instead move to a set of "reminders" on the ConversationListActivity.

Reminder about setting as default SMS will show up both on app start, and whenever TextSecure goes from being default to no longer default.

Reminder about import will only show up if TS is default and the user hasn't dismissed or imported yet.

![screenshot1](https://cloud.githubusercontent.com/assets/373823/3670582/a3fb1edc-1245-11e4-86f5-2d1acfeefbe2.png)
![screenshot2](https://cloud.githubusercontent.com/assets/373823/3670585/a62fae52-1245-11e4-832a-4f74e5215e79.png)