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 for getVoiceNetworkType() and currently
camped rat is GSM, they get network mode as GSM instead
of network mode unknown.

Change-Id: Ibe4672b04a5044710d63499c4cf2b6ebe91444bf
Signed-off-by: houst0nn <houstonn@gmail.com>