IMS-VT: Do not allow changing the TTY mode during a Video call

When the device is already in a video call, the user SHOULD NOT
be allowed to change the TTY mode. If the user tries to change,
then an UI alert message will be displayed explaining it cannot
be changed during video calls.

Change-Id: Id7fdeb7d90fbbdf94a6418eb58fd4bf00a5c241c
CRs-Fixed: 734258