Don't allow an external service to be an A11y service

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