Merge "Clarify meaning of untrusted_app and app domain assignment logic."

untrusted_app.te

 ###
 ### Untrusted apps.
 ###
-### This file defines the rules for untrusted apps. An "untrusted
-### app" is an APP with UID between APP_AID (10000)
-### and AID_ISOLATED_START (99000).
+### This file defines the rules for untrusted apps.
+### Apps are labeled based on mac_permissions.xml (maps signer and
+### optionally package name to seinfo value) and seapp_contexts (maps UID
+### and optionally seinfo value to domain for process and type for data
+### directory).  The untrusted_app domain is the default assignment in
+### seapp_contexts for any app with UID between APP_AID (10000)
+### and AID_ISOLATED_START (99000) if the app has no specific seinfo
+### value as determined from mac_permissions.xml.  In current AOSP, this
+### domain is assigned to all non-system apps as well as to any system apps
+### that are not signed by one of the four platform keys.  To move
+### a system app into a specific domain, add a signer entry for it to
+### mac_permissions.xml and assign it one of the pre-existing seinfo values
+### or define and use a new seinfo value in both mac_permissions.xml and
+### seapp_contexts.
 ###
 ### untrusted_app includes all the appdomain rules, plus the
 ### additional following rules: