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

android/proto.go

@@ -52,9 +52,8 @@ func ProtoDeps(ctx BottomUpMutatorContext, p *ProtoProperties) {
 	}
 
 	if plugin := String(p.Proto.Plugin); plugin != "" {
-		ctx.AddFarVariationDependencies([]blueprint.Variation{
-			{Mutator: "arch", Variation: ctx.Config().BuildOsVariant},
-		}, ProtoPluginDepTag, "protoc-gen-"+plugin)
+		ctx.AddFarVariationDependencies(ctx.Config().BuildOSTarget.Variations(),
+			ProtoPluginDepTag, "protoc-gen-"+plugin)
 	}
 }