Added configuration for VT downgrade/tear down when data disabled

Added a new flag that allows carriers to ignore data disabled
(e.g. data reaches limit or user disables data) so VT calls
will not be downgraded/torn down and VT service will not be
disabled.

bug: 32880745
Merged-In: Iad786b542c3db0cd300cef27a41e492e6888b639
Change-Id: Iad786b542c3db0cd300cef27a41e492e6888b639