neverallow debugfs access

Don't allow access to the generic debugfs label. Instead, force
relabeling to a more specific type. system_server and dumpstate
are excluded from this until I have time to fix them.

Tighten up the neverallow rules for untrusted_app. It should never
be reading any file on /sys/kernel/debug, regardless of the label.

Change-Id: Ic7feff9ba3aca450f1e0b6f253f0b56c7918d0fa