PowerWidget: Re-enable LTE button.

- Implement toggleLTE,
  from CM 10.0: I3f7521f5ec4e9f9106c58afd4d5b10c7086b55fd.
- Support all LTE network modes defined in RILConstants and
  com.android.internal.telephony.Phone.
- Cleanup: Remove unused LTE_MODE, restrict state change to updateState.
- Also use Settings.Global.getInt() for PREFERRED_NETWORK_MODE.