Move native-specific uploadSelectedMedia from updateImageUserAvatar to nativeUpdateUserImageAvatar

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

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

This diff also eliminates all usages of useUploadSelectedMedia, which means we can remove the prop in a subsequent diff at which point BaseEditUserAvatarProvider will be platform-agnostic! That'll let us consolidate BaseEditUserAvatarProvider, WebEditUserAvatarProvider, and NativeEditUserAvatarProvider.

Depends on D8345

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

Reviewers: ashoat, ginsu, rohan

Reviewed By: ashoat, rohan

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8346