RIL: fix signal for legacy gsm devices

our parcels are short causing mLteRssnr to be 0 which  is a valid
value and causes SignalStrength to think we are an LTE device.

re implement as oldRil "signalstrength"

Ported from Evervolv:
http://review.evervolv.com/#/c/4523/

Change-Id: Id1b47dbee6cc875bc2cde2f499f6ba788fa9ccb5