[10/n] Native Typeahead - SelectableTextInput for Android

Summary:
Putting this one up first, as it's the "base case". iOS basically has to do everything that Android does.

Depends on D6641

Test Plan:

• Tested both iOS and Android
• Made sure there was no flicker
• Tested @-tag in both middle and end of text
• Tested moving cursor after selecting @-tag

Reviewers: przemek, tomek, atul

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D6642