[native] Pull useShowAvatarActionSheet out of Edit*Avatar
Summary:
Pulling out some more logic that's in common between EditUserAvatar and EditThreadAvatar.
NOTE: One more quick refactor to tidy up editAvatarOptions and then we're gooing to move UserAvatar and ThreadAvatar "inside" Edit*Avatar components instead of just passing in React.Node.
Test Plan: Action sheet appears as it did previously + ESLint + Flow
Reviewers: ashoat, ginsu
Reviewed By: ashoat
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D7506