Refactor conference participant processing.

Moving the logic to handle transformation from an ImsConferenceState
instance to a List<ConferenceParticipant> to a standalone method.
This is to support testing conference event package testing.

Test: Manual conference event package regression testing.
Test: Test using conference event package test intents.
Bug: 150860282
Fixes: 154245604
Merged-In: I5bc84ef388fa3ecc6d6c8440a82a3104c06d56c0
Change-Id: I2343980511b85558e589fa01144688dcbc9a6071