Correct issues with post-SRVCC conference.

1. Do not remove PROPERTY_IS_DOWNGRADED_CONFERENCE from a merged
conference connection.  This ensures that the conference appears in the
UI as a conference call and not a standalone call.
2. Ensure that PROPERTY_REMOTELY_HOSTED is not set on the conference while
the SRVCC is in process.  This ensures that we do not log the conference
parent.

Test: manual; unit test
Bug: 194024590
Change-Id: Ie453d58208c78288c8d1ad985a1e0f00303ad16e
6 files changed