UserInfo atom

Adds a pulled statsd atom for UserInfo, containing each user's id, type,
and information about when it was last used.

We currently do not log the system user (user 0) because its information
is not currently interesting.

Test: make statsd_testdrive; statsd_testdrive <atom #>
Bug: 197545923
Bug: 218402402

Change-Id: Ic7187e5e093552e35c3628ea6c14ff1c2cd77613