Packet: Refactor packet fields definition

* Split GenGetter into GetGetterFunctionName() and GenGetter()
* Split GenBuilderParameter into GetBuilderParameterType and
  GenBuilderParameter()
* This allows other code generation routines to use builder types and
  getter function names separately

Bug: 143374372
Test: make and run unit tests
Change-Id: I748455dae0720df5c8e4aac9aedb64fc07660a12