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: Idd33a022b5a31cbc58592587bcb496bd78923ae7