[native] Make RegistrationContainer scrollable

Summary:
This diff does two things:

1. Makes RegistrationContainer scrollable, so if there isn't enough space in the viewport, the content can still be fully viewable
2. Add KeyboardAvoidingView to the RegistrationContainer, following the approach in Profile. This way when a TextInput is focused and the keyboard comes up, the ScrollView automatically adjusts so that the TextInput is still visible on screen

Depends on D7592

Test Plan:

(I also tested the same thing on Android)

Reviewers: ginsu, atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7593