Adds custom call forwarding block while roaming carrier config key

Adds support for supplying custom call forwarding prefixes in the carrier
config. When a prefix supplied in the carrier config is matched to the number
being dialed, we do not complete the call and instead show an error message
on the dialer UI.

Test: Manual
Bug: 30458052
Change-Id: I3149a565b0fd631d69ab0c0f27be509237b80177