Fixed the incorrect signal strength info displayed after call end.

We should query the signal stregnth information from the modem
when RAT family changes instead of relying on the next unsolicited
signal strength information indication coming from the modem,
which might take a long time to come or even not come. By proactively
querying the info, we can make sure the up-to-date signal strength
is showing up on the UI.

bug: 22724699
Change-Id: I4c80b281cffb28a871d4e754f43c43b9809244a4