untrusted_app lost the ability to read files labeled as sysfs to prevent
information leakage, but this is trivially bypassable by spawning an
isolated app, since this was not taken away from isolated app.
Privileges should not be gained by launching an isolated app, and this
one directly defeats that hardeneing. Remove this access.

Bug: 28722489
Change-Id: I61d3678eca515351c9dbe4444ee39d0c89db7a3e