Notification usage stats

First cut of gathering implicit notification signals and computing
running stats.

Tracks:
1. Post, update, remove by apps
2. Click, dismissal by users

Stats are aggregated on user, user+pkg, and getKey() levels.

Current stats are printed as part of 'dumpsys notification'.

Change-Id: I06ecbf76e517509895f2f9eea5b9d19bf9a34975