Reduce code duplication (EXPOSUREAPP-2844) (#1286)

* Remove code duplication between different flavors.
Remove extra MainFragment and NavGraph.
Introduce TestMenu indirection
Add logic for splitting test code up into extra fragments.

* Style list items a bit :)

* fix lints

* Detekt *hug*

* We can only show the menu entry if we are actually on the right flavor,
otherwise, in deviceDebug, we would not have access to the right resources.

* Address PR comments

* Fix wrong XML namespace.