[web] await baseSetThreadAvatar/baseSetUserAvatar in onImageSelected

Summary:
We could add a void here instead, but onImageSelected is already an async function, and the place we're passing it to doesn't await the result anyways. So might as well just add an await keyword.

Depends on D10174

Test Plan: Flow

Reviewers: atul, ginsu, michal

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10175