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
diff --git a/java/java.go b/java/java.go
index 4264ba9..47fb1a7 100644
--- a/java/java.go
+++ b/java/java.go
@@ -546,9 +546,7 @@
 	ctx.AddVariationDependencies(nil, libTag, j.properties.Libs...)
 	ctx.AddVariationDependencies(nil, staticLibTag, j.properties.Static_libs...)
 
-	ctx.AddFarVariationDependencies([]blueprint.Variation{
-		{Mutator: "arch", Variation: ctx.Config().BuildOsCommonVariant},
-	}, pluginTag, j.properties.Plugins...)
+	ctx.AddFarVariationDependencies(ctx.Config().BuildOSCommonTarget.Variations(), pluginTag, j.properties.Plugins...)
 
 	android.ProtoDeps(ctx, &j.protoProperties)
 	if j.hasSrcExt(".proto") {