Silence /proc/pid denials.

system_server components such as ActivityManager and CpuTracker
try to access all /proc/pid directories, triggering denials on
domains that are not explicitly allowed to the system_server.
Silence these denials to avoid filling the logs with noise
and overwriting actual useful messages in the kernel ring buffer.

Change-Id: Ifd6f2fd63e945647570ed61c67a6171b89878617
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>