ServiceManager: Check guaranteeClient before unregistering services

If tryUnregisterService is called while a service has set guaranteeClient to true, it should not succeed. The flag means that a client exists and is about to be counted. Not checking this flag can lead to a race.

Bug: 151485917
Test: aidl_lazy_test
Change-Id: If7ef7a5c7521ea40521bd351385fb8bd650aba08