Adds the snackbar functionality as we have for Posts uploads with Async (see #6445)

To test:

1. go to Media in the My Sites screen
2. tap on the "+" sign on the top-right corner of the screen to add a media item
3. choose anything (device video or photo, or take a new video / picture)
4. when the item starts uploading, you'll see the snackbar "Uploading media..." appear

5. when the item finishes uploading, you'll see the snackbar "Upload finished!"

cc @nbradbury