Add DORMANT state to GSM data link activity

GsmDataConnectionTracker sets its activity flag to DORMANT and notifies
LinkActivity when RIL indicates that all its data calls are in
dormant state (i.e. active is set to 1 in UNSOL_DATA_CALL_LIST_CHANGED
message for all data calls)

This indication is needed so that the Android public API getDataActivity()
can return DATA_ACTIVITY_DORMANT when Android applications invoke this
API while the phone is on a WCDMA data call.

Change-Id: I81b9db36375fea4f9fb946ddd5ddf0e1d16cbbab