libbinder: log when wrong number of threads start

Add a log when a service requests to have 0 threads but a threadpool
(with a thread) is still requested to be started). This currently
doesn't get hit on cuttlefish, but by triggering it manually I
can see the log.

Bug: 210145621
Test: trigger API misuse case and check for log
Change-Id: I9113828e10812895802a83017ed8f55e851fa220