Do not add dependencies from disabled sdk variants to its members

If an sdk variant is disabled (e.g. say windows) then it should not
add dependencies on corresponding sdk member variants as if those
variants have not been created then the build breaks unnecessarily.

This is needed to make progress on unbundling art which provides at
least one host tool (dex2oat) that is not supported on all platforms
and some (hiddenapi) that only produce 64 bit versions.
Initially, the art prebuilts will only support building 64 bit
versions of linux host tools.

Bug: 142935992
Test: m art-module-sdk art-module-host-exports art-module-test-exports
Change-Id: Idfbb40bb3cabc6eb731a2b38f045ed14f0a713cc