sepolicy: allow system_server to read /proc/locks

Access to /proc/locks is necessary to activity manager to determine
wheter a process holds a lock or not prior freezing it.

Test: verified access of /proc/locks while testing other CLs in the same
topic.
Bug: 176928302

Change-Id: I14a65da126ff26c6528edae137d3ee85d3611509
4 files changed