Mobile network type is not updated when the network mode is switched.

Because ServiceStateTracker doesn't handle the change of ServiceState
after switching of Telephony FW, SystemProperty used for displaying
the mobile network type does not become to Unknown. To fix this issue,
when the network mode is switched, the SystemProperty should be
initialized to Unknown in the constructor of ServiceStateTracker.

Change-Id: I903ba855b9fe22530db8494a9d2615959970c5a4