IServiceManager: add registerForNotifications.

This can help binder clients know when a service has been registered.
Potentially helpful, in situations where some HALs take some seconds to
come up and register with service manager.

Bug: 196432585

Test: manual
Test: binderLibTest

Change-Id: Ib1d9b967a7cffa99f43dc26f9025caf1f59ab77f
Signed-off-by: Jayant Chowdhary <jchowdhary@google.com>