Packet: Refactor packet generation executable

* Split packet parsing and header generation into two separate functions
  such that the parsing results can be used separately later
* Use Google style for functional arguments:
  - const reference for input
  - pointers for output

Bug: 143374372
Test: make, gd and packet unit tests
Change-Id: Id8ff4e805dc8e1b21dde50bdafdd8cf9bba36998