Add a carrier config indicating whether user can edit a voicemail number

Some carriers require to make it possible to prevent user editing
voicemail number. This carrier config is used to disable voicemail
number setting in Settings.

Test: manual - Checked that the voice mail number editing could be
controlled by carrier config.
Bug: 67872671

Change-Id: Ibbc3e0aa9d3d15abbb44477acd6d3546593a21bf