keyguard: Handle left icon visibility properly with wifi only devices

When telephony isn't available and the left icon has not yet been set to
a custom option, the left icon should be hidden.

This makes updateLeftAffordanceIcon and updateLeftButtonVisibility work
in a similar way to determine the icon visibility.

Change-Id: Ibb63ecef3c3cad73a19c04c85eccb93b423d5e80