Move namespace mutator earlier to support defaults

The prebuilts and defaults mutators start adding some dependencies, so
for them to be able to reference imported modules, we need to
run the namespace mutator earlier.

Test: m nothing
Test: Try to use a global defaults module from a namespace
Change-Id: I6e853d20e32251d0fd33c8b2dcc26c0695b808fd
diff --git a/android/namespace_test.go b/android/namespace_test.go
index 9ab186b..a6fc9d5 100644
--- a/android/namespace_test.go
+++ b/android/namespace_test.go
@@ -606,7 +606,7 @@
 	ctx.MockFileSystem(bps)
 	ctx.RegisterModuleType("test_module", ModuleFactoryAdaptor(newTestModule))
 	ctx.RegisterModuleType("soong_namespace", ModuleFactoryAdaptor(NamespaceFactory))
-	ctx.PreDepsMutators(RegisterNamespaceMutator)
+	ctx.PreArchMutators(RegisterNamespaceMutator)
 	ctx.Register()
 
 	_, errs = ctx.ParseBlueprintsFiles("Android.bp")