Add method to determine variations from a Target

The arch variants are hardcoded in every module type.  Refactor
them out into a Target.Variations() method in preparation for
splitting the arch mutator into two, which will require using
different variations.

Test: m checkbuild
Change-Id: I28ef7cd5168095ac888fe77f04e27f9ad81978c0

sdk/sdk.go

@@ -101,11 +101,10 @@ func memberMutator(mctx android.BottomUpMutatorContext) {
 
 		targets := mctx.MultiTargets()
 		for _, target := range targets {
-			mctx.AddFarVariationDependencies([]blueprint.Variation{
-				{Mutator: "arch", Variation: target.String()},
+			mctx.AddFarVariationDependencies(append(target.Variations(), []blueprint.Variation{
 				{Mutator: "image", Variation: "core"},
 				{Mutator: "link", Variation: "shared"},
-			}, sdkMemberDepTag, m.properties.Native_shared_libs...)
+			}...), sdkMemberDepTag, m.properties.Native_shared_libs...)
 		}
 	}
 }