Move native-specific uploadSelectedMedia from updateImageThreadAvatar to nativeUpdateImageThreadAvatar

Summary:
We remove the native-specific uploadSelectedMedia call from updateImageThreadAvatar and move it to the native-specific nativeUpdateThreadImageAvatar.

As of this diff, updateImageThreadAvatar now takes (imageAvatarUpdateRequest: ImageAvatarDBContent) rather than the native-specific (selection: NativeMediaSelection).

This diff also eliminates all usages of useUploadSelectedMedia, which means we can remvoe the prop in a subsequent diff at which point EditThreadAvatarProvider will be platform-agnostic.

Test Plan: Continue to be able to set image thread avatars on native as expected.

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8772