lmkd: grant read access to all of /sys

avc: denied { search } for pid=394 comm="lmkd" name="lowmemorykiller" dev="sysfs" ino=7541 scontext=u:r:lmkd:s0 tcontext=u:object_r:sysfs_lowmemorykiller:s0 tclass=dir permissive=0

Bug: 29558514
Change-Id: Iaae907a92976af2a9dcb58be5643b8614dcde174