Add new telephony disconnect cause for video calls when tty enabled.

Due to the way the radio works, it isn't possible to make a video call
when tty is enabled.  Adding a new telephony disconnect cause to handle
this case.
Note: This was previously handled directly in Telecom which is not
correct.

Bug: 20090652
Change-Id: I90b7d6486fe00c84efe7bb1c0f30edf33a1502bc