Make cross-user apps mlstrustedsubject.

We have various apps which inherently work across all users,
configured in seapp_contexts with levelFrom=None (usually implicitly).

This change marks those apps, where they have private data files, as
mlstrustedsubject, to allow us to increase restrictions on cross-user
access without breaking them.

Currently these apps are granted full access to [priv_]app__data_file
via TE rules, but are blocked from calling open (etc) by mls rules
(they don't have a matching level).

This CL changes things round so they are granted access by mls, but
blocked from calling open by TE rules; the overall effect is thus the
same - they do not have access.

A neverallow rule is added to ensure this remains true.

Note that there are various vendor apps which are appdomain,
levelFrom=None; they will also need modified policy.

Test: builds, boots, no new denials.
Bug: 141677108

Change-Id: Ic14f24ec6e8cbfda7a775adf0c350b406d3a197e
diff --git a/private/bluetooth.te b/private/bluetooth.te
index 1680361..0453b3d 100644
--- a/private/bluetooth.te
+++ b/private/bluetooth.te
@@ -1,6 +1,6 @@
 # bluetooth app
 
-typeattribute bluetooth coredomain;
+typeattribute bluetooth coredomain, mlstrustedsubject;
 
 app_domain(bluetooth)
 net_domain(bluetooth)