Enable EMERGENCY mobile connections.

Hooks the ConnectivityManager.TYPE_MOBILE_EMERGENCY,
PhoneConstants.APN_TYPE_EMERGENCY, and NetworkCapabilities.NET_CAPABILITY_EIMS
together so carrier apps can request connections to this APN.

bug:21785357
Change-Id: Id92a5e28d19407cc7a8f8b5478b23457f2f7f89d