keyguard: SIM Display Name is now a config

In devices with more than one SIM, keyguard visually distinguishes each SIM
card by showing its color and its name.

This patch introduces now a configuration item that lets you choose wether or
not show the SIM display name on those cases.

Issue: PAELLA-224
Change-Id: I8fb019d4fa534b2ca2f047083c27aa9cf8b7b39c
(cherry picked from commit 6c769a70)