[native] Pull useUploadProcessedMedia out of Edit[User/Thread]Avatar
Summary: Pull out common functionality from Edit[User/Thread]Avatar and move to newly introduced avatar-hooks.js.
Test Plan:
- Replace onPressEmojiAvatarFlow with openPhotoGallery in Edit[User/Thread]Avatar
- Ensure that selected image appears in keyserver uploads table as before.
Reviewers: ashoat, ginsu
Reviewed By: ashoat
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D7493