Move drawable out of inset state, only store constant state

This ensures that InsetDrawable's constant state only references other
constant states rather than holding an entire Drawable, which provides
a clean separation between local and constant state information. This
ensures that calls to mutate() can create a new constant state without
losing local drawable state and that newDrawable() still relies on
the same constant state.

Bug: 19156549
Bug: 13877782
Change-Id: I48146f6c7ac3593009cfa323fc341bcd7d8ab21b