Deduplicate and rationalize system_server /proc/pid access.

The system_server has duplicate/overlapping rules regarding
/proc/pid access as well as a lack of clarity on the reason
for the different rules.  Deduplicate the rules and clarify
the purpose of different sets of rules.

Replace the rules granting /proc/pid access for all domains with
specific rules only for domains that we know should be accessible
by the system_server, i.e. all apps (appdomain) and the set of
native processes listed in com.android.server.Watchdog.NATIVE_STACKS_OF_INTEREST.

Change-Id: Idae6fc87e19e1700cdc4bdbde521d35caa046d74
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>