Navigator.push allows us to open a new screen. Currently it starts a new Activity with the screen
as it’s root view, but this is only a temporary behaviour. The intended behaviour is similar to
View Controllers on iOS which allows you to manage view hierarchies or a stack of fragments
managed by FragmentManager on Android.

This class handles all Android specific logic. Currently only startTabBasedApp is implemented,
but in the future all interactions with the UI will go here.

The approach we took is fairly simple. Navigation is created natively in corresponding activity,
currently only TabActivity is supported, and we pass the styling parameters and screenId to it.
Each screen is rendered in its own ReactRootView and is wired to the same ReactInstanceManager,
so all screens share the same java script context.