Move cobalt config dart library target to separate file

Dart dependencies only exist on topaz and onwards so including the gni
file in lower layers fails. This moves the dart dependency and the
target that's generated using the dart templates to a separate file that
must only be imported in topaz and upwards.

Change-Id: Ic809f1fd3b2bb6d0e04facd27b604b319002e0a9
2 files changed