Add Dual Transfer Mode support.

In GsmServiceStateTracker, decode the "concurrent services support
indicator" field from RIL_REQUEST_VOICE_REGISTRATION_STATE
request and based on the value of this CSSI field allow concurrent
CS and PS services to applications.

Change-Id: Iebf43ed14ff38d88b06f7bb43d11365de6433b81
CRs-Fixed: 564063