Convert a missed Singleton

Convert EnvSingleton to android.SingletonFactory.

Test: m checkbuild
Change-Id: Ia18b0b97718ba7c08d3f136f6f4096477d90c0f4
diff --git a/android/env.go b/android/env.go
index ec5794e..c03431b 100644
--- a/android/env.go
+++ b/android/env.go
@@ -19,8 +19,6 @@
 	"strings"
 
 	"android/soong/env"
-
-	"github.com/google/blueprint"
 )
 
 // This file supports dependencies on environment variables.  During build manifest generation,
@@ -43,13 +41,13 @@
 	os.Clearenv()
 }
 
-func EnvSingleton() blueprint.Singleton {
+func EnvSingleton() Singleton {
 	return &envSingleton{}
 }
 
 type envSingleton struct{}
 
-func (c *envSingleton) GenerateBuildActions(ctx blueprint.SingletonContext) {
+func (c *envSingleton) GenerateBuildActions(ctx SingletonContext) {
 	envDeps := ctx.Config().(Config).EnvDeps()
 
 	envFile := PathForOutput(ctx, ".soong.environment")