Use relative paths to //mojo in EDK BUILD.gn files

As a step toward making the Mojo EDK flexible wrt its location in a client
repo, this CL changes Mojo EDK targets that are of type "component" and "test"
to specify paths within the Mojo SDK/EDK as relative rather than absolute. As
there are few targets of type "component" and "test", we have decided against
making templates as we are doing for source_set.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/759143005