servicemanager: notifications

You can now register and receive notifications for new services.

Eye towards:
- avoiding while(true) getService loops
- need it to make AIDL dynamic HALs
- need it for feature parity w/ HIDL

Bug: 136027762
Test: servicemanager_test
Change-Id: Iffbf984e87214aa9c9b7af8a5ae682e127ba40a5