[native] Pull useSelectAndUploadFromGallery out of Edit[User/Thread]Avatar

Summary:
Pull out common functionality from Edit[User/Thread]Avatar and move to newly introduced avatar-hooks.js.

Added entry to editAvatarOptions in both components to consume this hook.

Depends on D7497

Test Plan:

1. Click on pencil icon
2. Ensure that the system media gallery appears as expected
3. Ensure that the selection is processed as expected (logging + breakpoint)
4. Ensure that selected image appears in keyserver uploads table:

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7498