Don't create version variants for SDK variants

When a lib has sdk_version set, an SDK variant and a platform variant
are created by the sdkMutator. Then by the versionMutator, if the
library had 'stubs.versions' property, one or more versioned variants
and one impl variant are created for each of the two (SDK and platform)
variants. As a concrete example,

cc_library {
    name: "foo",
    sdk_version: "current",
    stubs: { versions: ["1", "2"], },
}

would create 6 variants:

1) (sdk: "", version: "")
2) (sdk: "", version: "1")
3) (sdk: "", version: "2")
4) (sdk: "sdk", version: "")
5) (sdk: "sdk", version: "1")
6) (sdk: "sdk", version: "2")

This is somewhat uncessary because the need for the SDK mutator is to
have the platform variant (sdk:"") of a lib where sdk_version is unset,
which actually makes sens for the impl variant (version:""), but not
the versioned variants (version:"1" or version:"2").

This is not only unncessary, but also causes duplicate module
definitions in the Make side when doing an unbundled build. Specifically,
The #1 and #4 above both are emitted to Make and get the same name
"foo".

To fix the problem and not to create unnecessary variants, the versioned
variants are no longer created for the sdk variant. So, foo now has
the following variants only.

1) (sdk: "", version: "") // not emitted to Make (by versionMutator)
2) (sdk: "", version: "1") // not emitted to Make (by versionMutator)
3) (sdk: "", version: "2") // emitted to Make (by versionMutator)
4) (sdk: "sdk", version: "") // not emitted to Make (by versionMutator)

Bug: 159106705
Test: Add sdk_version:"minimum" to libnativehelper in libnativehelper/Android.bp.
m SOONG_ALLOW_MISSING_DEPENDENCIES=true TARGET_BUILD_UNBUNDLED=true libnativehelper

Change-Id: I6f02f4189e5504286174ccff1642166da82d00c9