Mutate and apply theme if needed before caching themed drawables

This ensures that drawables are completely separated from their cached
constant states before applying a theme. After this, we can remove the
implicit (and incomplete) mutation in the clone constructors.

Also implements missing mutate() method on ClipDrawable.

BUG: 17646144
Change-Id: If0d66b0a85724d76e0a4f506758c7ba3c0aa3410