Add preparer for overlay pre-singleton registration

It appears as though this is the first pre-singleton type to actually
be registered with the InitRegistrationContext as it failed due to an
uninitialized map, so this change also fixes that.

Bug: 182885307
Test: m nothing
Change-Id: Ibbf6d0db5f3c2fcc89291a16aa5f16b8b5009bd3
diff --git a/java/testing.go b/java/testing.go
index 4b8b849..2343294 100644
--- a/java/testing.go
+++ b/java/testing.go
@@ -54,6 +54,8 @@
 	android.FixtureAddTextFile(defaultJavaDir+"/Android.bp", GatherRequiredDepsForTest()),
 )
 
+var PrepareForTestWithOverlayBuildComponents = android.FixtureRegisterWithContext(registerOverlayBuildComponents)
+
 // Prepare a fixture to use all java module types, mutators and singletons fully.
 //
 // This should only be used by tests that want to run with as much of the build enabled as possible.