Switch from DEPs to git submodules?

For historical reasons Amber was setup using `DEPS` and `tools/git-sync-deps`. The Chromium project has been moving to submodules from DEPS, which is where Amber picked up using DEPS from.

Should we switch from DEPS and add submodule entries for each of the things in there? I think it would make things simpler for users and catch us up with how other DEPs based projects are proceeding.