Add carrier config KEY_CALL_FORWARDING_WHEN_UNREACHABLE_SUPPORTED_BOOL

Adds carrier config KEY_CALL_FORWARDING_UNREACHABLE_SUPPORTED_BOOL.
Call forwarding option "When Unreachable" in Call Settings will be
disabled and greyed if this value is set to false.

Default value is true, meaning that the option will be available for
most carriers.

Bug: 118861990
Test: Manual
Change-Id: Ib1e525d6c3983f0675c1730defaa18832c504a43