Am I the only one who's disappointed by Google's UI library chops? How can they think it is sensible to release a UI design without having concrete, usable implementations in all the major UI frontends, including bootstrap, out of the box? The fact that someone outside of Google had to build this makes it look like Google's design guys have no credibility inside of Google, and are ignored by other Google devs.
Android L is also shipping with Material Design widgets: https://developer.android.com/preview/material/index.html
Google doesn't really use or promote Bootstrap (much... we're* so big I'm sure it's used somewhere), and Bootstrap is designed for a pre-web components, pre-flexbox world, so it shouldn't be too surprising that we didn't release a theme for it.
* I work for Google