Add telephony.DisconnectCause for merged IMS call.

When merged, IMS calls are consolidated into one call. Thus they
"disconnect", even though the user can continue to comminucate
with the party on that call.

We want to recognize this scenario so we can control relevant
behaviors (ie.don't play the "end call" tone in this case).

Bug: 18291234
Change-Id: I93247f88ed74467470504bec88fcfb7ed714f0ff