Support theme attributes in StateListDrawable <item> element

Also adds support for specifying drawable as a color since getDrawable()
automatically wraps colors to ColorDrawable. Does not currently allow
themed item elements to be used in Zygote preload, but we can add that
at some point in the future.

BUG: 18208662
Change-Id: I4c9721ffd100da4b9db7743a46c914828b943dae