logd: refine permissions to access /data/system/packages.list

Refine DAC security surrounding logd.daemon worker thread and add a
positive test for logd failure to access /data/system/packages.list.

- Add AID_PACKAGE_INFO to groups of worker thread.
- Move AID_SYSTEM to groups, setgid to AID_LOGD.
- Do not drop capabilities until after setting the uid and gids.
- Add a test that is part of logd.statistics test to check when
  packagelistparser appears broken.
- If /data/system/packages.list is encrypted, ensure we do not pick
  up the existing inode to ensure strong positive when finding access
  problems.
- Replace all occurrences of NULL with nullptr in gTest code for
  compliance with best practices.

Test: gTest logd-unit-tests --gtest_filter=logd.statistics
      (expect consistent failure, later CLs fix)
Bug: 37751120
Bug: 36645158
Change-Id: I01b26fe5e25203246ae432d272c8daa9c07cab54