Create --unsigned flag to create unsigned builds

This change is designed to allow us to create production builds without
a dependency on Keystore, with the ultimate goal of automating release
builds.  With this change we can perform post-build signing which should
drastically reduce the amount of time required to create a release.

Some details can be found at http://go/gf-cpe-signing

Change-Id: Ibb2f954248cba3c2464dae107ba0f0982fd7646c
7 files changed