Update ConnectionService#addConferenceFromConnection to properly cleanup the conn.

This CL ensures onConnectionRemoved is signaled for the original Connection when ConnectionService#addConferenceFromConnection is called. This prevents a regression that was occurring where remote conferences were errantly not being considered holdable since the topHoldableCount was hitting 2.

Test: atest ImsCallingTest
Fixes: 445215865
Flag: com.android.internal.telephony.flags.reuse_original_conn_remote_conf_behavior
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:eabf2e9a069e8a5be5c8d5d9372af11d81cb7ee2
Merged-In: Ife1649671808b113243c2bb55f0f38562e33cac8
Change-Id: Ife1649671808b113243c2bb55f0f38562e33cac8