Only request image and version variations for device SDK dependencies

AddFarVariationDependencies was broken, which allowed sdk modules to
request dependencies using image and version variations, even for
host modules that do not have image or version variations.  Make
the image and version variations conditional on device sdk modules.

Test: go test ./sdk
Change-Id: I59b7a32a3782254fd5feb828a5258ee13d4db812