AlertDialog owns its view, which causes problems with TabLayout on API 21
and 22. Now using AppCompatDialog instead so that we can own the view
ourselves.
Also:
- Renamed layout files from download_xyz to track_selection_xyz.
- Added OK and Cancel buttons to the view.
- Applied alert dialog style to the "normal" dialog to get a nicer UI.
PiperOrigin-RevId: 233944647
