IMS-VT: Support IMS calls fallback to CS

1. Display alert dialog to seek user confirmation.
   If confirmed for a retry, then initiate a cs call.
2. In case of silent redial, automatically initiate the cs call.

Change-Id: I0b0d9880fa017ab91cff9dfe2e292237d73c0d18
CRs-Fixed: 781748
4 files changed