Switch app_* and isolated to _app and _isolated in seapp_contexts.

The app_* syntax was a legacy of the original approach of looking up
the username returned by getpwuid() and the original username encoding
scheme by bionic.  With the recent changes to move away from this approach,
there is no reason to retain that syntax.  Instead, just use _app to match
app UIDs and _isolated to match isolated service UIDs.  The underscore
prefix is to signify that these are not real usernames and to avoid
conflicts with any system usernames.

Requires a corresponding change to libselinux.

Change-Id: Ic388a12c1c9d3e47386c8849db607140ef8a3d75
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>

seapp_contexts

@@ -8,6 +8,8 @@
 # An unspecified isSystemServer defaults to false.
 # An unspecified string selector will match any value.
 # A user string selector that ends in * will perform a prefix match.
+# user=_app will match any regular app UID.
+# user=_isolated will match any isolated service UID.
 # All specified input selectors in an entry must match (i.e. logical AND).
 # Matching is case-insensitive.
 # Precedence rules:
@@ -33,10 +35,10 @@ isSystemServer=true domain=system
 user=system domain=system_app type=system_data_file
 user=nfc domain=nfc type=nfc_data_file
 user=radio domain=radio type=radio_data_file
-user=app_* domain=untrusted_app type=app_data_file levelFromUid=true
-user=app_* seinfo=platform domain=platform_app type=platform_app_data_file
-user=app_* seinfo=shared domain=shared_app type=platform_app_data_file
-user=app_* seinfo=media domain=media_app type=platform_app_data_file
-user=app_* seinfo=release domain=release_app type=platform_app_data_file
-user=app_* seinfo=release name=com.android.browser domain=browser_app type=platform_app_data_file
-user=isolated domain=isolated_app levelFromUid=true
+user=_app domain=untrusted_app type=app_data_file levelFromUid=true
+user=_app seinfo=platform domain=platform_app type=platform_app_data_file
+user=_app seinfo=shared domain=shared_app type=platform_app_data_file
+user=_app seinfo=media domain=media_app type=platform_app_data_file
+user=_app seinfo=release domain=release_app type=platform_app_data_file
+user=_app seinfo=release name=com.android.browser domain=browser_app type=platform_app_data_file
+user=_isolated domain=isolated_app levelFromUid=true