Fix two cosmetic issues in signapk.

* Zip EoCD record comment length was referred to as 32-bit whereas it
  is a 16-bit field. The implementation was fine, but the comment and
  the naming of a constant were wrong.
* System.out.println was left over from early prototyping days.
  Removed.

Bug: 25794543
Change-Id: I97199310d4b4451271a75bb6c6d0463e0b788be9