Updates to Telecom API docs for clarity.

- Update Connection onAnswer/onReject docs to make it more clear for
a self-managed connectionservice that these methods could be called by
the framework or by a car mode in call service.
- Update the Connection onShowIncomingCallUI method to take into account
the new concurrent calling capabilities in P.

Test: Docs only change
Bug: 73112552
Change-Id: Icb47aa15d03d4bfc05380948e30686317124301e