Add utility for manipulating CallForwardInfo.

Part of the motivation in this is to be able to provide clearer
function names and constant names for magical operations. It seemed
like to CallFeatureSettings (Voicemail Settings), these details
could be encapsulated on their own.

Bug: 17019623
Change-Id: Ie064cc0d47fb47643145562936ce8feadc4bec49
2 files changed