Only register mutators once

Mutators are registered into a global variable, and then into the
context when it is created.  Only call registerMutators once so that the
same mutator is not listed multiple times in the global variable when
multiple contexts are created.

Test: go test -v android/soong/android
Change-Id: Ie9e3ed09a89b848462b898476cdfb81a90c64bd3

android/register.go

@@ -14,7 +14,11 @@
 
 package android
 
-import "github.com/google/blueprint"
+import (
+	"sync"
+
+	"github.com/google/blueprint"
+)
 
 type moduleType struct {
 	name    string
@@ -47,6 +51,8 @@ func RegisterSingletonType(name string, factory blueprint.SingletonFactory) {
 	singletons = append(singletons, singleton{name, factory})
 }
 
+var registerMutatorsOnce sync.Once
+
 func NewContext() *blueprint.Context {
 	ctx := blueprint.NewContext()
 
@@ -58,7 +64,7 @@ func NewContext() *blueprint.Context {
 		ctx.RegisterSingletonType(t.name, t.factory)
 	}
 
-	registerMutators()
+	registerMutatorsOnce.Do(registerMutators)
 
 	for _, t := range mutators {
 		var handle blueprint.MutatorHandle