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

1. Add LTE signal strength (ie: rxlev, rsrp and rssnr) on emulator.
2. Fix range check for rssi/ber at handleSignalStrength@android_modem.c

"gsm lte_signal rxlev rsrp rssnr" can be used to simulate the
LTE siganl strength on emulator with this patch.