Framework: Fix GSM signal strength

Certain OEM gsm ril libs send back a bogus value for LTE signal
that causes SignalStrength.java to default to LTE and report
incorrect signal values.
Use prop value ro.telephony.ril.v3=signalstrength to get proper
GSM signal reading on certain devices.

Signed-off-by: dhacker29 <davidhackerdvm@gmail.com>

Change-Id: I86279dcbc350fe718d89070acc12178de3c600c2
telephony: SignalStrengh: Don't calculate a LTE snr if the srq is invalid

If the previous values are invalid and signal quality is -1, chances are there's
no LTE signal at all, so don't calculate the SNR and mark it invalid as well.

This fixes the strength indicator in GSM devices where it was being skewed
by the ghost LTE value

Change-Id: I070573401c141aa99941a332ee256246697abb90