Store SingletonMakeVarsProviders in the config

Store SingletonMakeVarsProviders in the config instead of a global
variable to avoid races between tests running in parallel.

Test: all soong tests
Change-Id: I2ab64f368b5ac673fd985399d4421ed018abc562
diff --git a/android/testing.go b/android/testing.go
index d83cecc..1e2ae13 100644
--- a/android/testing.go
+++ b/android/testing.go
@@ -104,7 +104,7 @@
 }
 
 func (ctx *TestContext) RegisterSingletonType(name string, factory SingletonFactory) {
-	ctx.Context.RegisterSingletonType(name, SingletonFactoryAdaptor(factory))
+	ctx.Context.RegisterSingletonType(name, SingletonFactoryAdaptor(ctx.Context, factory))
 }
 
 func (ctx *TestContext) ModuleForTests(name, variant string) TestingModule {