Add activity indicator in image blocks

As commented here: https://github.com/wordpress-mobile/gutenberg-mobile/pull/447#issuecomment-449974645

It would be nice to add an activity indicator to the loading of images. The current implementation looks like this (downloading):

![image_loading](https://user-images.githubusercontent.com/9772967/50441050-928f6880-0901-11e9-9691-1c709c4f80db.gif)
