Adds stats for:
- Number of raw user events that have happened in the system.
- Number of times user activity has been reported, dividied by UID and type of activity.
- Duration of screen brightness levels in 4 buckets.
- Per-UID tracking of who has turned on Wifi and how long we can attribute it being on because of them.
BUG=1743326
Automated import of CL 143748
65c8e11a31