1137 - fix crash on tablets

Fix for #1137 

* Default the signal strength and CDMA dbm to be UNKNOWN_SIGNAL
* if no telephony hardware is detected, the CellScannerImplementation::start() short circuits and doesn't enable any listeners
* stop() on the CellScannerImplementation will only unhook the listener if it's non-null and the telephony type is GSM or CDMA.