Telephony: Add support for NETWORK_TYPE_GSM

Add support for NETWORK_TYPE_GSM in TelephonyManager and
corresponding mapping in ServicState.java so that when
application queries getVoiceNetworkType() and currently
camped rat is GSM, they get network mode as GSM instead
of network mode unknown.

Change-Id: I5e971c625532696776b9555ad8cfd85eba4fc423
CRs-Fixed: 526435