Nullify stl and sanitize of llndk_headers

llndk_headers can't depend on other non-llndk cc modules such as libc++,
just like llndk_library. This nullifies stl and sanitize to prevent such
dependencies.

Bug: 160639583
Test: m nothing, see dependencies of llndk_headers
Change-Id: I3c6830d6b46754981c4ae10a7e7e013ec849efaf

cc/llndk_library.go

@@ -225,6 +225,8 @@ func (headers *llndkHeadersDecorator) Name(name string) string {
 func llndkHeadersFactory() android.Module {
 	module, library := NewLibrary(android.DeviceSupported)
 	library.HeaderOnly()
+	module.stl = nil
+	module.sanitize = nil
 
 	decorator := &llndkHeadersDecorator{
 		libraryDecorator: library,