"not open-source in the sense of being a community project." Android is no less a community project than FF OS. And it does have a great community of outside developers around it, XDA. Check out the forums.

If you want to contribute back, Google has been good about accepting patches into its AOSP project. For patches they won't accept, there's always CyanogenMod.

"JS engines are pretty fast" Good enough for simple text based apps and casual games, yes. Modern 3D games, not by a long shot. Native code, NDK, or Java under HotSpot are the only decent options creating for modern responsive 3D games.