libril: Provide support for legacy ril

With change Icc620fd191f8542f61bd38e365813753bfedefdf, RIL_VERSION_MIN was
increased from 2 to 6, but this causes problems for a number of devices
using ril blobs older than that.

  E/RILC    (  140): RIL_register: version 3 is to old, min version is 6

Consequently causing the ril system unable to connect to the rild socket.