Excluded certain APNs (e.g. IMS) from mobile data usage.

Added not_metered capability to a mobile network if none
of its associated APN types are metered. Also used not_metered
capability to determine if a network should be accounted for
data usage or not instead of using network type, which is
always MOBILE after refactoring. Will add VT usage support
in next phase.

bug: 20888836
Change-Id: Ic39c9ffc84e27566a5b10ebf9aaa4ee3d6bd3e49