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.config=signalstrength to get proper
GSM signal reading on certain devices.

Commit includes "telephony: hide needsOldRilFeature" from Chirayu Desai.

Change-Id: I9a0b68f6c3789218a4a20337496e6264996deab7