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