Support Animated VectorDrawables

[benpious]

Android supports this. We may want to support it too.

We'll need to investigate whether this is actually a popular enough feature of the Android SDK to justify support, as Lottie is already an excellent cross platform option for those who need this feature.

The main blocker to this is that it will likely require a new API that allows loading off the main thread but can return the size of the drawable immediately, as animated drawables will likely be significantly larger than static ones, but we should be able to return something that has a size immediately. This may imply a more complex data structure than the VectorDrawable struct that can be passed around while it's still deserializing off the main thread, then can notify its displaying view (if it has one) once it's done.

[flatslats]

+1 for this feature!