Minor improvements on SystemServerInitThreadPool:

- Log number of threads on constructor.
- Use explicit, private lock object so callers cannot accidentally cause dead
  locks.
- Add a new method to explicitly start it.
- Let the whole object be GC'ed when done.
- Don't let tasks be submitted after it's been shutdown.

Bug: 141571552

Test: manual verification

Change-Id: Ibff5f014e17366ab2eec9b64da00c9416d98fc54