There is a race condition in the root navigatior's initialization.

It's possible that it's initialized a touch too late and SDK users who
try to navigate to a conference end up stuck in the connecting screen
because the navigator is null.

This PR waits for it to be initilized by very unorthodox means, it's a
horrible hack which we need to undo, but for that we need to break
appart the inheritance relationship between App.{web,native},
AbstractApp and BaseApp because it's very inflexible.

The flags are now initialized very early so the naviggator sees if the
welcome page is enabled or not.