Grant untrusted_app dir access to asec_apk_file.

untrusted_app lost all of the domain_deprecated permissions in N,
including the ability to read asec_apk_file dirs.  This is used for
forward locked apps.

Addresses the following denials:
avc: denied { search } for name="asec" dev="tmpfs" ino=9298 scontext=u:r:untrusted_app:s0:c512,c768 tcontext=u:object_r:asec_apk_file:s0 tclass=dir permissive=0
avc: denied { getattr } for path="/mnt/asec" dev="tmpfs" ino=9298 scontext=u:r:untrusted_app:s0:c512,c768 tcontext=u:object_r:asec_apk_file:s0 tclass=dir permissive=0

(cherry-pick of internal commit: addd3c9f)

Bug: 30082229
Change-Id: I87758f1daee19197d9299bca261f0324e01af5e0