gril: Add RIL voice call parcels handling

Functions for packing/parsing RIL requests, replies, and unsolicited
events, for voice call related cases.
rilmodem: Re-factor voicecall code

Re-factor code to ease unit testing and OEM adaptation if needed.
unit: Add tests for gril voicecall functions
Merge pull request #28 from alfonsosanchezbeato/voicecall-refactor

Voicecall refactor