fix(client): Do not show Fx for Android marketing material if completing sign up on B2G or Fennec.

This is a straw man until we decide whether we really need to hide marketing material if the user starts on Fennec or B2G. This PR hides marketing material if the user completes on Fennec or B2G. Marketing material is still shown if the user is in a desktop browser or a tablet where the user is not using Fennec (and media queries do not hide the material, of course).

issue #1041