Check upgrade certs before permissions.

We're now checking for permission redefinition early during the
install process, which can result in a confusing error message when
the real problem is a mis-signed app.  So do a quick signature
sanity check before checking permissions.

Bug: 18095637
Change-Id: I9a9b48da9c5dc7fb9bde6f3f338ea08e53b6b705