Fix setting handed-over calls to ANSWERED state

When a handover happens, CallsManager calls answerCall on it after the
handover completes (i.e. when it's already in the active state). Add a
check for the call being in the ringing state before marking it as
ANSWERED so that ACTIVE -> ANSWERED transition doesn't happen.

Bug: 111822381
Test: manual, unit
Change-Id: I19a2ffa948e5f968d0c10a3eba9bf817a77348ee