Split bugreport identifier into id and pid.

Although 'pid' is more useful when diagnosing problems with the
bugreport workflow, it could be confusing to the end user. Hence, a
sequential id (started at 1 after a reboot) would be more useful.

BUG: 27076108
Change-Id: I6fd7ea8c703e8040034af183ef5cd35adfbcb4ef