Telephony: Add getLteOnGsmMode() method (1/2)

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.
Fix UMS build

Change-Id: I8a186b7379e218086cab4debad297b68cd7022a4