LazyServiceRegistrar: race w/ register & onClients

This is similar to a fix in HIDL. In AIDL, we have more control over
when a threadpool starts, so this race is only possible when someone
configures their services after starting the threadpool:
- a process has a threadpool of size > 1
- it calls register service (but hasn't recorded internally that the
  service is registered)
- another process gets ahold of the service
- servicemanager tells the service it has a client
- the service aborts, because it has no record of registering this
  service

Bug: 191608065
Test: aidl_lazy_test
Change-Id: I9e61a1c958d82035d49eb84e6ae71dba00019c43