When some applications create an external service and register this service to be an A11y service, then this service can kill the system and restart it continuously. This is a security vulnerability and we add not allow an external service to register as an A11y service to fix it. Bug: 216115176 Test: a11y CTS & unit tests Change-Id: If75b9c300df73e98be92d6c14d69301cc377b205