Add a new platform_bootclasspath module type

Initially, this is just a placeholder but functionality will be added
in follow up changes.

Bug: 177892522
Test: m nothing
Change-Id: I890b0d5a117c51a19c9ac5df98c766761d3aa16c
diff --git a/java/testing.go b/java/testing.go
index 1113af7..80c107d 100644
--- a/java/testing.go
+++ b/java/testing.go
@@ -194,6 +194,7 @@
 	RegisterDocsBuildComponents(ctx)
 	RegisterGenRuleBuildComponents(ctx)
 	registerJavaBuildComponents(ctx)
+	registerPlatformBootclasspathBuildComponents(ctx)
 	RegisterPrebuiltApisBuildComponents(ctx)
 	RegisterRuntimeResourceOverlayBuildComponents(ctx)
 	RegisterSdkLibraryBuildComponents(ctx)