Add a new SingletonModule type

A SingletonModule is halfway between a Singleton and a Module.  It has
access to visiting other modules via its GenerateSingletonBuildActions
method, but must be defined in an Android.bp file and can also be
depended on like a module.

Bug: 176904285
Test: singleton_module_test.go
Change-Id: I1b2bfdfb3927c1eabf431c53213cb7c581e33ca4

android/testing.go

@@ -103,6 +103,12 @@ func (ctx *TestContext) RegisterModuleType(name string, factory ModuleFactory) {
 	ctx.Context.RegisterModuleType(name, ModuleFactoryAdaptor(factory))
 }
 
+func (ctx *TestContext) RegisterSingletonModuleType(name string, factory SingletonModuleFactory) {
+	s, m := SingletonModuleFactoryAdaptor(name, factory)
+	ctx.RegisterSingletonType(name, s)
+	ctx.RegisterModuleType(name, m)
+}
+
 func (ctx *TestContext) RegisterSingletonType(name string, factory SingletonFactory) {
 	ctx.Context.RegisterSingletonType(name, SingletonFactoryAdaptor(ctx.Context, factory))
 }