Telephony: Add getLteOnGsmMode() method (2/2)

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

From CM 10.0: Ibfb47ca608e51393b99d3308e0a6c66050b3f32e
- Moved getLteOnGsmModeStatic() to TelephonyManager.