Confine shell domain in -user builds only.

Confine the domain for an adb shell in -user builds only.
The shell domain in non-user builds is left permissive.
init_shell (shell spawned by init, e.g.  console service)
remains unconfined by this change.
Introduce a shelldomain attribute for rules common to all shell
domains, assign it to the shell types, and add shelldomain.te for
its rules.

Change-Id: I01ee2c7ef80b61a9db151abe182ef9af7623c461
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>