Fix bug of getRingingCall

It returns the ringing call of ImsPhone if the ringing call of GSMPhone
isn't ringing. In CallManager.registerPhone(), it always registers ringing
call of ImsPhone, because the ringing call of GSMPhone isn't ringing.
Consequently, it can't answer GSM call successfully by invoking
TelephonyManager.answerRingingCall() since the implementation in
PhoneInterfaceManager.answerRingingCallInternal() could not get the correct
ringing call from CallManager.

We check the ringing call state of imsPhone first as accpetCall() does.