The denial message:
update_engine: type=1400 audit(0.0:15213): avc: denied { getattr } for
path="/postinstall" dev="dm-0" ino=38 scontext=u:r:update_engine:s0
tcontext=u:object_r:postinstall_mnt_dir:s0 tclass=dir permissive=0

update_engine: type=1400 audit(0.0:15214): avc: denied { sys_rawio } for
capability=17 scontext=u:r:update_engine:s0 tcontext=u:r:update_engine:s0
tclass=capability permissive=0

auditd  : type=1400 audit(0.0:15213): avc: denied { getattr } for
comm="update_engine" path="/postinstall" dev="dm-0" ino=38
scontext=u:r:update_engine:s0 tcontext=u:object_r:postinstall_mnt_dir:s0
tclass=dir permissive=0

update_engine: [0428/070905:ERROR:utils.cc(716)] Error stat'ing /postinstall: Permission denied

Bug: 37760573
Test: apply an update and UE reads postinstall_mnt_dir without denial.
Change-Id: I55506f5e8544233f60ccf7c1df846c9c93946a25