Allow adbd to pull sepolicy from device.

Address the following denial when running CTS:
avc: denied { search } for comm=73657276696365203136 name="/" dev="selinuxfs" ino=1 scontext=u:r:adbd:s0 tcontext=u:object_r:selinuxfs:s0 tclass=dir permissive=0

Bug: 26290097
Change-Id: Icf51061a65a5fda15f7e7ef78200a62ffbd1ca9b