Add a setting to overrid emergency number for Emergency SOS

- Add a dialog preference and custom dialog to allow user input a phone
  number
- Use settingslib/emergencynumber util to set/get the user input into
  SettingsProvider

https://screenshot.googleplex.com/A9GmJrcSbp4jYfd.png

Bug: 172609395
Test: robotests
Change-Id: Iedd9bb9f32c212a310fd8fddffad852a997f3b0d