IMS: Support for Possible Call Session Handover

Adding the callSessionMayHandover API that allows
for the lower layers to indicate that the call session
may handover based on actions taken by the user/UI
layer.
The API provides current and target RAT information
pertaining to the possible handover.

Bug: 62489988
Test: Manual
Change-Id: I006462ce5688bb99a94d5135574fb2c17878685a