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.