Media: Add ability to pull to refresh (sync) media library

[rachelmcr]

A user noted in an app review that if you add media remotely, the media library in the app doesn't automatically sync with the remote library:

Does not update automatically
Like when I added an image from my desktop computer and make a post using this app adding the image on the post, the recently uploaded image does not display. I have to go back to home, click on media and refresh it.

It would be great to have the ability to pull to refresh (sync) the media library content from within the media library picker in the editor, the same as you can in the Media section.

Tested on Nexus 9, Android 7.0, WPAndroid alpha-28