[user_shell][armadillo] Get UserShellContext from the environment for armadillo.

This sets us up to be able to delete the UserShell interface entirely,
which existed solely to pass dependencies to user shells. This is not
necessary.

TESTING=none

MI4-1445 #comment [user_shell] Get UserShellContext from the environment.

Change-Id: I04c0409319278c37a57771cc19204d5f5bebbdd2
3 files changed