Add the custom FAB to AndroLabsApp file

[RivanParmar]

Note: This task should be done only after #7 is closed.

• The custom FAB should be added to the AndroLabsApp file in the Scaffold.
• The FAB should be positioned above the Navigation Bar on small screen devices, while it should be present above the Navigation Rail items on large screen devices.
• The variables shouldShowBottomBar and shouldShowNavRail available in the AndroLabsAppState can be used to position the FAB accordingly.