Bug 850996 - B2G Emulator: Add LTE signal report function. r=vicamo.

1. Add LTE signal strength (ie: rxlev, rsrp and rssnr) on emulator.
2. Fix range check for rssi/ber at handleSignalStrength@android_modem.c.
3. print rssi/ber if no argument for "gsm signal" command,
   print rxlev/rsrp/rssnr if no argument for "gsm lte_signal" command.

Ex:
1. "gsm signal" will print the current rssi/ber on console.
2. "gsm lte_signal" will print the current rxlev/rsrp/rssnr on console.
3. "gsm lte_signal <rxlev> <rsrp> <rssnr>" will be used to set the
   LTE siganl strength on emulator, and report RIL_UNSOL_SIGNAL_STRENGTH
   to upper layer.