Use LTE SNR and RSRP to set signal level bar.

The LTE signal strength level is the smaller one
between lte rsrp level and lte snr level if both
rsrp and snr are valid.

The lte snr mapping are

  Four  bars: SNR >= 45
  Three bars: 10 <= SNR < 45
  Two   bars: -30 <= SNR < 10
  One   bars: SNR < -30
  No    bars: No Service

The invalid value of lte snr is changed to INVALID_SNR
from -1, since -1 is a valid value of lte snr.

bug:5640958
Change-Id: If26aaba0c7fcc0fee3db488b5adfa02922f06715