Telephony: Add getLteOnGsmMode() method

Same functionality as the existing getLteOnCdmaMode, but for GSM
LTE devices.
Enable with the telephony.lteOnGsmDevice system property

Additionally, support GSM LTE in the SystemUI LTE power widget

Change-Id: Ibfb47ca608e51393b99d3308e0a6c66050b3f32e