DO NOT MERGE: Allow shell debugfs read access

Developers should be able to use systrace with user builds.
This requires read access to /sys/kernel/debug/tracing/trace,
otherwise the following error occurs:

  $ atrace
  capturing trace... done
  TRACE:
  error opening /sys/kernel/debug/tracing/trace: Permission denied (13)

with the following SELinux denial:

  <4>[   79.830542] type=1400 audit(11940551.039:8): avc:  denied  { read } for  pid=1156 comm="atrace" name="trace" dev="debugfs" ino=3024 scontext=u:r:shell:s0 tcontext=u:object_r:debugfs:s0 tclass=file

At least on the kernel I've tested this on, debugfs doesn't support
setting SELinux file labels. Grant read access to all of debugfs to
work around this limitation.

Bug: 13904660
Change-Id: Ic494bfccafc8f6887c8a4c8278b91245459aea41