Bug 914599 - Improve Call Settings when FDN is enabled

When the Fixed Dialing Numbers options is enabled, many features are
disabled, and USSD/MMI codes fall into this set. This will thus make the
many call settings parameters getting into error state. The present
commit aims at: avoiding undefined state of the call settings, and
improving the feedback we send to user by clearly stating that call
forwaring options are unavaiable due to FDN enabled.
Merge pull request #12158 from lissyx/bug914599

Bug 914599 - Improve Call Settings when FDN is enabled, r=kaze