Self Managed CS Implementation

- Add API guide for developers giving an overview of what they need
to implement.
- Add @hide setLabel method in PhoneAccount; used when registering a
self-managed CS's phone account; we override it with their app label to
ensure they don't try to give themselves another name.

Test: Unit
Bug: 34159263
Change-Id: I0c890dc2feeb3ee438a80ed425db2aaf1f4c1fdd