APEXs are signed with apk signer

The entire APEX (which is a zip file) is signed with the apk signer.
Certificate can be specified via the 'certificate' property just like
ordinary apps. Note: multiple additional certificates are not supported.

Bug: 115721587
Test: m apex.test
Test: jarsigner -verify -verbose -certs .../apex.test.apex shows the
certificate info

Change-Id: Ia4c898d3427779a3809fdc683b85d7661ca65137