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
(cherry picked from commit 3e01fcec13725955f0ca3fb20730ff21f4edc128)
(cherry picked from commit c70f5687965908cd585e52901cf6cd88bd405140)
(cherry picked from commit 8e3c2300729ecb3cd3e6b640b0174f49e2189f0c)