Allow zygote to scan static overlays on /oem

During preloading resources, zygote scans the overlay directories of
supported partitions looking for android RROs to apply statically. Zygote
currently is allowed to read overlays in /oem/overlay, but zygote does
not have the search permission to be able to scan /oem.

Without this patch, this denial is logged:
04-04 14:57:40.136   876   876 I auditd  : type=1400 audit(0.0:9):
avc: denied { search } for comm="main" name="oem" dev="dm-3" ino=46
scontext=u:r:zygote:s0 tcontext=u:object_r:oemfs:s0 tclass=dir
permissive=0

Bug: 121033532
Test: booting without denials and stat oem succeeds
Change-Id: I661f3e0aff7ec3513870d08ddc122fc359b8f995
1 file changed