Don't look for verifier/installer when core only.

When booting the device in core-only mode we're running over a tmpfs
and only a small handful of apps are actually available.  Don't
bother looking for verifiers or installers, since we probably won't
find them.

Bug: 26438767
Change-Id: Ia4c4a6e71007b01c4aaa273c2e0197b5cd021262