Migrate Android Shell over to using all the same platform structures as non-shell

- Takes us a step closer towards https://github.com/dotnet/maui/issues/6389. WinUI already uses shared code between Shell and Non Shell.
- Massively reduces maintenance burden by obsoleting all existing android specific shell code.
- Performance. The newer structures we are using for FlyoutPage/NavigationPage perform slightly better than the current shell platform code. Plus this reduces the amount of code we have to tune through AOT profiles and keep track of as it relates to ensuring code paths stay performant.

No files, tests, or entry points are named. Begin by locating the Android-specific Shell code and the shared structures already used by non-Shell code, then compare the WinUI shared-code approach; done means Android Shell uses the common platform structures and the obsolete Android-specific Shell code is removed.

Written by the indexing model from the issue text.