Fixes #2

As mentioned in #2, a ctor was missing for the `PictureBitmapDrawable`, I think this was added with 4.8 if I recall correctly, where derivatives of Java types need to have this ctor.

I also added some guards as sometimes the `Rect` passed into `OnBoundsChange` have their width and height set to 0, which crashes the app. This happens when using the `Drawable` in a `Fragment` and the `ImageView` hasn't been measured or drawn yet.