Add disconnect failed connection event

Adds a new Telecom connection event to signal a disconnect failure when
a hangup operation is performed during a call merge.

Bug: 409455144
Bug: 406274501
Flag: com.android.server.telecom.flags.revert_disconnecting_during_merge
Test: atest CallTest
Change-Id: I63cac32d068a3e89f9ce3d8d6eaa5edd9d084329