Use relative paths for deps in the group targets in //mojo/public.

This is a step on the way toward making the BUILD.gn files under //mojo/public
flexible with respect to the location of the public SDK in a client repo. As
there aren't enough group targets to justify making a mojo_sdk_group template,
this CL simply changes absolute paths to relative paths.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/783433003