It's a CTS requirement that all SELinux domains be in
enforcing mode. Add the same assertion to the build system
when targeting user builds.

In particular, this avoids a situation where device integrity
checking is enabled on user builds, but permissive denials
are being generated, causing the device to unexpectedly reboot
into safe mode.

A developer wanting to put an SELinux domain into permissive
mode for userdebug/eng purposes can write the following
in their policy:

  userdebug_or_eng(`
    permissive foo;
  ')

Bug: 26902605
Bug: 27313768
Change-Id: Ic0971d9e96a28f2a98f9d56a547661d24fb81a21