bootstat: switch from root.root to system.log

(cherry picked from commit c3ad75be)

bootstat does not need root uid and root gid permissions to perform
its tasks.  It appears that system uid and log gid are adequate and
appropriate.

Test: manual
Bug: 63736262
Bug: 65843095
Change-Id: I094c2cb054e441562fa8717a4d3dc0086fb70a7a