Unregister data enabled changed listener on default phone

[Description]
"com.android.phone" process crash after ACTION_IMS_SERVICE_DOWN
is sent out when ImsPhone is ready.

[Root cause]
When ACTION_IMS_SERVICE_DOWN is sent out, ImsPhone is disposed
and ImsPhoneCallTracker unregisters data enabled changed listener
in DcTracker from instance of ImsPhone but ImsPhone has no
object of DcTracker(i.e. null) then NullPointerException
has been happend.

[Solution]
When ImsPhoneCallTracker unregisters the listener, it refers
to default GsmCdmaPhone which it has an object of DcTracker

NOTE:
When ImsPhoneCallTracker registers the listener, it already refer
to default GsmCdmaPhone.

Test: manual with adb command to send IMS related intent.
Bug: 34206487
Change-Id: Iac9636de33074634fe9ccd451d510b4863bdc9ba