Android: Refine app flow, begun fragmentalization

- App flow has now been refined a bit.
- Fragmentalization had begun.
- App now uses the MainActivity just to check and see whether the user
is logged in or not. If not, it moves to the login activity, which then
moves to MainFragmentActivity after login. If so, it moves to the
MainFragmentActivity which is where the fragments (once they are
converted from activities) will be displayed.
- Tidied up files a bit.