Workaround for LTE RSRP -ve values in Signal

Ril version 6 and below sends negative values
for lte rsrp in signalstrength.
Workaround to maintain backward compatibility
and convert this to positive
values in compliance with ril.h

Support for processing all fields in
SignalStrength in reference-ril

Change-Id: I4d480aa2f607cd617b18681f722de246a7894348