IMS-VT: Unregister CallManager from ModifyCallRequest

CallManager needs to unregister the UNSOL_MODIFY_CALL
request when it unregisters all phone events.

Change-Id: I0d383f8b406a69670f1e69890e5b635346c2657f
CRs-Fixed: 587612
Allow ECT MMI Command to be dialled in Alerting State

MMI Commands are not allowed to dialled if there
is a Foreground Call with Dialing/Alerting State.
Allow ECT MMI Command (4+Send) in Alerting state
as per spec 3GPP TC 31.13.1.4 Requirement

Change-Id: I9dcc07dc31e05462dce0c34dfa2bd8e4e347b709
CRs-Fixed: 585553

Conflicts:
	src/java/com/android/internal/telephony/CallManager.java
	src/java/com/android/internal/telephony/ExtCallManager.java