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.

Test: telephony unit tests
bug: 32880745
Change-Id: I8852d1f636d7cc73802119666b902b7df6a39e72