TeleService: Fix issue can't answer/reject call when at USSD content view

When show the USSD dialog, MT a call to the device at this time and
the IncallScreen will show behind the USSD dialog. That's because
the USSD dialog has the WindowManager.LayoutParams.TYPE_SYSTEM_DIALOG
and WindowManager.LayoutParams.FLAG_DIM_BEHIND properties.
So it always show on top of screen.

We should add the function to hide the USSD dialog when a new ringing
connection comes and show it again after disconnect this connection.

CRs-Fixed: 551995

Change-Id: I5ea315c012ecd17f8d0e5e3b36299a80d6f20fc8