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 sepolicy.
Change-Id: I21f9f88415b653c1bf6332fc100d91d969c9da64 Signed-off-by: Stephen Smalley <email@example.com>