Add active services count callback to lazy services.

Additionally, expose methods to tryUnregister/reRegister
services.

Bug: 176239128
Test: test aidl_lazy_test

Change-Id: I9aa3c9b681bd340ca340fe7ed818ba3533678af2
Merged-In: I9aa3c9b681bd340ca340fe7ed818ba3533678af2