Ensure call direction for CEP participants is unknown to start.

Ensure the call direction property for CEP participants starts as unknown;
we will update it based on cached call data later.

Test: Run all unit tests.
Test: Manual testing using VoLTE conference calls with mix of MO and MT
calls; verify call logging is appropriate.
Bug: 134471046

Change-Id: I15a173fb6685dc78e14ebe817c53bb7cfc236eaf