Convert a missed Singleton

Convert EnvSingleton to android.SingletonFactory.

Test: m checkbuild
Change-Id: Ia18b0b97718ba7c08d3f136f6f4096477d90c0f4
diff --git a/android/testing.go b/android/testing.go
index f12c032..60189d3 100644
--- a/android/testing.go
+++ b/android/testing.go
@@ -61,7 +61,7 @@
 func (ctx *TestContext) Register() {
 	registerMutators(ctx.Context, ctx.preArch, ctx.preDeps, ctx.postDeps)
 
-	ctx.RegisterSingletonType("env", EnvSingleton)
+	ctx.RegisterSingletonType("env", SingletonFactoryAdaptor(EnvSingleton))
 }
 
 func (ctx *TestContext) ModuleForTests(name, variant string) TestingModule {