Telephony: Query SignalStrength when there is a RAT change.

In CdmaLte phone when RAT switch happens, telephony receiving
new signal info before updating to new RAT and because of that
the received signal info is ignored and empty bars updated
on UI. To avoid that query signal strength info when there
is a change in PS RAT happens.
CRs-Fixed: 464438

Change-Id: Ia8fbca8260cce64978ad7ad88a9963d8c598d0ed