Clarify meaning of untrusted_app and app domain assignment logic.

The current inline documentation is not entirely accurate and caused
user confusion, e.g. see:
https://groups.google.com/d/msg/android-security-discuss/javBrPT8ius/C4EVEFUu4ZoJ



Try to clarify the meaning of untrusted_app, how app domains are
assigned, and how to move other system apps out of untrusted_app into
a different domain.

Change-Id: I98d344dd078fe9e2738b68636adaabda1f4b3c3a
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>