[native] Pull selectFromGallery/useUploadSelectedMedia out from EditThreadAvatarProvider

Summary:
As part of making EditThreadAvatarProvider "platform-agnostic" so it can be used across web and native, we're pulling out selectFromGallery and useUploadSelectedMedia out of EditThreadAvatarProvider.

This is the thread avatar version of D8215

Depends on D8218

Test Plan: Made sure I could still successfully select/upload image thread avatar from native.

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8219