[native] Set RegistrationTextInput keyboardAppearance based on theme

Summary:
For the registration flow, this will force dark mode since the registration flow can only be triggered in dark mode.

For other uses of RegistrationTextInput, this will set the keyboardAppearance to match the rest of the app's theme.

Depends on D8000

Test Plan: Tested to make sure that the keyboardAppearance was getting set correctly

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8001