Update "one" key's voicemail status if it changed

When launching the Phone app, it checks whether a voicemail
number is available and sets the background image of the "one"
key accordingly.
If a voicemail number was set *after* the app was launched,
the button was never updated (unless the app was force-closed
manually).

Change-Id: I6ef18cbb1a72609ebdd3380d2ebd76a986b8295b