[native] Return popoverLocation from useReactionSelectionPopoverPosition

Summary:
We'll need this in a later diff

Depends on D7010

Test Plan: Flow, and testing the reaction popover animation on iOS and Android release builds, including opening the emoji keyboard

Reviewers: ginsu

Reviewed By: ginsu

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7011