[rilmodem] Enable context deactivation when RoamingAllowed is set to FALSE

Without this change context is left active if only RoamingAllowed is called
in ConnMan API when there is active context. Connection cannot be used
but it is misleading from user point of view and also agains ofono spec.
Therefore it is better enable deactivation.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Merge pull request #89 from jkangas/master

Request for pulling context deactivation based on RoamingAllowed setting to main branch