IMS: Detach disconnected connections from its parent

Detach disconnected connections from its parent once
the call is ended

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IMS: After SRVCC handover, disconnect the dropped IMS calls

After a successful SRVCC handover, disconnect the handover pending
or dropped IMS calls, if any

Bug: 18777284
Change-Id: If4899c70c587c70b8202559b3fe0027ecd166f21