Post / Media : Adding image from media library needs to show loading status

[bummytime]

Original issue: https://github.com/wordpress-mobile/WordPress-Android/issues/230

On a slow connection, it looks like nothing is happening on the media picker screen if the media library has not already been cached.

[rachelmcr]

This is still an issue with the media picker screen in the new editor. If the media library has not already been cached, on a slow connection it looks like nothing is happening. I ended up with a list of blank white thumbnails and no sign that the app was loading those images:

Tested on Nexus 9, Android 6.0.1, alpha-4.