Make Connection#onSilence a public API.

Connection#onSilence is generally applicable to apps implementing the
self-managed ConnectionService API.
Also updated the docs to make it more clear where that API is to be used
and how the developer can silence the ringtone.

Test: CTS test, manual test.
Bug: 110348674
Change-Id: I1c1791c101827780949fd633c531ed83037e7b4e