Add new job scheduler constraints for "battery not low".

Clean up the implementation of boolean constraints so that
they are in a single flags value that gets propagated all of
the way from JobInfo.Builder in to the JobStatus.  Much simpler
and easier to add new constraints!

Also introduce some shell commands to make it easier to write
tests against the job scheduler (and other things tied to power).
One of the big things here is that there is a new sequence number
that propagates with battery updates, which we can look for in
the job scheduler to determine when a change the test has made
to battery state has actually gotten applied, to allow it to
safely and minimally wait until executing the condition being
tested.

Test: New BatteryConstraintTest suite added.

Change-Id: I11076d90b80ec25ee604c29b6a6dc2337fd867ce