Only show "Emergency calls only" if a GSM device is voice capable.

This fix uses the config_voice_capable compile time switch to be
sure GsmServiceStateTracker.mEmergencyOnly is false if the device
isn't voice capable. Thus GsmServiceStateTracker.updateSpnDisplay
will not send the SPN_STRINGS_UPDATED_ACTION intent with
emergency_calls_only as the PLMN. Instead, it will aways be the
string defined by the lockscreen_carrier_default, typically
"No service".

Bug: 9422921
Change-Id: I74e89c4a29d0d8e3d5cc1726adedb0c2634cf257