Add preparer for test with variables

Bug: 181070625
Test: m nothing
Change-Id: I101826d88e67c63a58007f20f8e24d574dd7c99b
diff --git a/android/variable.go b/android/variable.go
index dd000ad..be12a0a 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -24,11 +24,17 @@
 )
 
 func init() {
-	PreDepsMutators(func(ctx RegisterMutatorsContext) {
+	registerVariableBuildComponents(InitRegistrationContext)
+}
+
+func registerVariableBuildComponents(ctx RegistrationContext) {
+	ctx.PreDepsMutators(func(ctx RegisterMutatorsContext) {
 		ctx.BottomUp("variable", VariableMutator).Parallel()
 	})
 }
 
+var PrepareForTestWithVariables = FixtureRegisterWithContext(registerVariableBuildComponents)
+
 type variableProperties struct {
 	Product_variables struct {
 		Platform_sdk_version struct {