lmkd: allow lmkd to lock itself in memory

addresses the following denial:

  type=1400 audit(1.871:3): avc:  denied  { ipc_lock } for  pid=1406 comm="lmkd" capability=14  scontext=u:r:lmkd:s0 tcontext=u:r:lmkd:s0 tclass=capability

Bug: 16236289

(cherry picked from commit 6a1405d7)

Change-Id: I560f1e52eac9360d10d81fc8a9f60eba907a8466