Work towards issue #26390161: Throttle syncs/jobs when system is low on RAM

First, we need to make the job scheduler prioritize jobs for
foreground apps over background apps (so we will degrade well
when we are limiting the number of concurrent jobs).

So now the job scheduler keeps track of the process state of
each uid, and uses that to bump up the priority of jobs
associated with foreground uids.  Added constants for priorities
since we have different places specifying priorities.

Also cleaned up a bit of the reporting of "wrapped" jobs from
the sync manager -- there is a new tag argument that can be supplied,
to have the name and tag used in various places be based on that
instead of the useless internal class name.

Change-Id: I8781750ddfac1472a98e1873fc38c014425db3d6