Allow untrusted_app access to temporary apk files.

Before actual installation, apks are put in a staging area where they are
scanned by a verifier before completing the install flow.  This verifier runs as
a priv-app, which is in the untrusted_app domain.  Allow untrusted_app
read-access to these files.

Bug: 16515815

Change-Id: Ifedc12a33b1f53b62f45013e7b253dbc79b02a4e