Allow system_server pstore access.

pstore contains /sys/fs/pstore/console-ramoops, which is the
replacement for /proc/last_kmsg. Both files are read by system_server
on startup. Allow access.

Addresses the following denials:

<12>[   53.836838] type=1400 audit(949060020.909:19): avc:  denied  { search } for  pid=1233 comm="Thread-119" name="/" dev="pstore" ino=10296 scontext=u:r:system_server:s0 tcontext=u:object_r:pstorefs:s0 tclass=dir
<12>[   53.856546] type=1400 audit(949060020.909:20): avc:  denied  { getattr } for  pid=1233 comm="Thread-119" path="/sys/fs/pstore/console-ramoops" dev="pstore" ino=10297 scontext=u:r:system_server:s0 tcontext=u:object_r:pstorefs:s0 tclass=file
<12>[   53.878425] type=1400 audit(949060020.909:21): avc:  denied  { read } for  pid=1233 comm="Thread-119" name="console-ramoops" dev="pstore" ino=10297 scontext=u:r:system_server:s0 tcontext=u:object_r:pstorefs:s0 tclass=file
<12>[   53.898476] type=1400 audit(949060020.909:22): avc:  denied  { open } for  pid=1233 comm="Thread-119" path="/sys/fs/pstore/console-ramoops" dev="pstore" ino=10297 scontext=u:r:system_server:s0 tcontext=u:object_r:pstorefs:s0 tclass=file

Change-Id: I7307da751961b242e68adb319da9c00192e77bbb