Bundle public keys with APEX

When an apex key is marked as 'installable: false' and the build is
debuggable, the pubic key file for the apex key is bundled with the APEX
that is signed with the key.

This eliminates the need to install the public keys for the
testing-purpose APEX in the system partition.

Bug: 122047804
Test: m
Change-Id: Ifa5914891463dbf4c21484ea440836521b2f90b1