[native] Add native picker button to MediaGalleryKeyboard

Summary:
Resolves https://linear.app/comm/issue/ENG-2560/build-component-above-keyboard-image-photosbrowse-all-photos-that

Figma design: https://www.figma.com/file/L675ETKDnGaSwlpZAw4MIC/Mobile-App?node-id=3972%3A53851&t=63ZWpVfkfvm4AHad-0

Depends on D6495

Test Plan:
This is how it looks:

Reviewers: ashoat, inka, przemek, ginsu

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6497