Add a way to disable run-as at runtime

This change adds the ro.boot.disable_runas system property, that when
set, disables the run-as command. This is done to reduce the surface
area of programs that have file based capabilities in Chrome OS, and
what they can do when running in non-developer mode.

Bug: 31630024
Test: run-as still works in aosp_sailfish
Test: run-as still works in Android in Chrome OS (in developer mode)
Change-Id: Iaf1d6f9ceb65081b7a9e17b9b91d8855e4080133