[native] Create the toggle pin modal

Summary:
When pressing the icon in the tooltip, a modal should pop up on the screen allowing the user to pin / unpin a message. This diff handles the modal creation, but to make it easier to review, the
actual rendering of the message in the modal will be in the next diff following this one.

I placed the modal in app-navigator.react.js since the TextMessage component requires the OverlayContext to be set, so when rendering the Message > TextMessage inside of this modal in the next diff,
we won't have the invariant requiring this context be thrown.

Note: The approach here on native follows closely the approach for web here: https://phab.comm.dev/D7309

Depends on D7606

Test Plan:
Confirm that the modal appears (not rendering the message yet) for messages and multimedia, both that should be pinned and unpinned.

Reviewers: atul, ginsu, tomek, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D7607