Fix issue where incoming rejected calls were not being marked as such.

Code in onDisconnect for ImsPhoneConnection assumed that if the current
disconnect cause is "LOCAL" that we couldn't ever override this value.
However, in the case of an incoming rejected call, we do want to override
the disconnect cause.

Bug: 30474601
Change-Id: I3ffe4e6844658e063c01ea5e3aeec82989d053cd