Be smarter about determining when we can stop idle maintenance.

The device idle service now knows when the system is actively
doing significant things (syncs, jobs, alarms, downloads).  It
uses this, when in an idle maintenance window, to determine when
it can end that window early -- when such work is no longer
happening.

For now this just allows us to shorten the windows.  In the future
we should use this to allow us to expand the windows to a longer
potential time, adjusting future windows to shorter durations if
earlier ones use more time.  This will allow us to batch occasional
long operations (such as downloads) into one window, making up
for that with much shorter later windows.

Change-Id: Ie482abd50bc43be9a8917a769a5175851eee4ec4