Prevent NullPointerException in SimStatusDialogController

In rare cases the SignalStrength we get for a SIM can be null, which
wasn't checked for in this code. This CL adds a null check and fixes the
tests so it's possible to verify the fix (one of the methods that would
have crashed was being spy'd over unnecessarily).

Fixes: 132570076
Test: make RunSettingsRobotests
Change-Id: Ifdec24f184ee2d93e5e242eb2c20695249992fc6