[RTT2] RTT Responder config mods to meet API requirements

Convert class and members to final - per API guidelines. Add a generic
constructor for all arguments (which will never be unhidden) to support
an intermediate service-side stage of the Responder config.

Bug: 65108607
Test: unit tests, integration tests
Change-Id: Icbcbd8259856c8290bbc005f5905578da90febbc