Refactor container activities

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#254

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Again, #357 should be merged first to stay sane when reviewing this PR.

There's one major roadblock that's preventing me from proceeding: Twitter only lets you receive onActivityResults from an activity so there are three options:

1. Make more breaking api changes 😞 that would require devs to send us the onActivityResult from their activity.
2. Make two IdpSignInContainers: one that's an activity for SignInDelegate and one that's a fragment for AuthMethodPickerActivity. (This means that when people are restoring their phones they will see grossness because smart lock will probably be enabled, but other than that, normal sign-in will be pretty) Prefered option?
3. Don't refactor IdpSignInContainerActivity at all (probably a bad idea)

Thoughts?

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Done refactoring EmailHintContainerActivity. It turned out kinda gross so any thoughts would be appreciated.

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I just realized #418 replaces this PR.