Abstract an `SMSProvider` to allow GoogleVoice or TextNow or whatever

[patcon]

TextNowPlus should also support GoogleVoice. (VoicePlus is actually a bad name, and the original, Babel, might be better.)

This setting screen should be more than a dialog, but a new view where provider can be selected:

Settings > Wireless & networks > Voice+

Choosing Google Voice as provider will allow you to select your Google account. Choosing TextNow will allow you to enter a client_id value for now. This is only available via snooping for now. System apps might have the ability to read shared_prefs from other apps, if we can figure out how to decrypt the client_id value that TextNow stores. Otherwise, sniffing traffic is the simplest way. We could also release a version of the app, meant to be installed temporarily, that would output the client key in a toast or notification. Lastly, the best option would be if the client key was stored as a credential using Android's AccountManager, but that would require support of TextNow devs.

Possible provider options

• [x] Google Voice
• [x] TextNow (via unofficial api)
• [x] Ting (decompilation shows no SMS API in app)
• [x] Twilio (SMS and SIP support possible, but presumably needs developer key)
• [ ] Hushed app
• [ ] UppTalk app

[patcon]

Created a ticket in CM issue queue, as that's where VoicePlus lives: https://jira.cyanogenmod.org/browse/CYAN-6472