Move constants from ApfFilter to ApfConstant class for maintainability

Moved constants from ApfFilter into the ApfConstant class to:
* Improve code consistency and reduce duplication between ApfFilter and
  its tests.
* Facilitate future refactoring of ApfFilter into independent function
  blocks.
* Enable potential future merging with NetworkStackConstant, after
  addressing naming conflicts and potential relocation of
  NetworkStackConstant(from Connectivity module to NetworkStack module).

Test: TH
Change-Id: I7e9bab0db13b7b4cb1384681b06fbceb07847a34
2 files changed