Nsdmanager/NsdService: add logging

This patch adds basic logging to NsdManager and NsdService, and improves
the facilities for pretty printing the event ids defined in NsdManager.

It also includes a few minor cleanups:
  - adding 'final' on effectively final instance variables of NsdManager
  and NsdService.
  - similarly, adding 'static' on effectively static class fields.
  - regrouping instance variables together.

Test: no functional changes
Bug: 33074219
Change-Id: I360d539e73cc8e4b45d4e0d20b2e345455fdb10c