Bug 970212 - B2G Emulator: Enhance GSM/UMTS Signal Strength on Emulator. r=Vicamo.

0. revise based on the v5 comment.
1. Add handleSignalStrength to parse signal strength info to RIL_SignalStrength_v6.
2. Add an unsolicited +CSQ handling to let "gsm signal" command work on emulator.
3. remove un-necessary RIL_onRequestComplete(t, RIL_E_GENERIC_FAILURE, NULL, 0); at
   requestSignalStrength due to already handle it at error scope.