API cleanups per API review.

1. removing deprecation for setExtras on Connection/Conference as these
have legitimate use-cases going forward.
2. Expanded documentation on Call.Callback class to clarify how it is
expected to be used.
3. Updated Callback#onConnectionEvent callback docs to clarify its usage.
4. Expanded Call#sendCallEvent docs to clarify expected namespacing of
call event keys and extras keys, as well as clarification about backwards
compatibility and expectations of ConnectionServices.
5. Standardized the "invoked" vs "notifies" language in the Conference
docs.
6. Clarified how Connection callbacks from Telecom should be implemented.
7. Added Bundle parameter expectations for pre-defined connection events.
8. Documented capabilities and properties toString methods.
9. Documented expectations for onCallEvent method.
10. Expanded documentation for sendConnectionEvent method to clarify
namespacing, use cases, etc.

Bug: 29759275
Bug: 29759954
Bug: 29759773
Change-Id: Ib8c983af4dbbfee68edb60937b268e2c1a9b5e64