Initial implementation of light-weight idle mode.

This mode turns on after the screen has been off for 15 minutes,
and then cycles through 15 minutes of idle and 1 minute of
maintenance, ragardless of whether the device is moving around.

It currently only impacts network access and sync/job scheduling.
It does not remove access to wake locks or alarms for any apps.
It also doesn't report in the public API that the device is in
idle mode (since it isn't modifying the behavior of the power
manager) -- this is probably what we desire, since we don't want
stuff like GCM to be reporting these frequent changes.

We'll probably at least want to have the alarm manager do some
kind of more aggressive batching of alarms in this most (not allowing
more than one wakeup every minute?).  That's for the future.

Also updated batterystats to include this new information, which
means the format of some of the data has changed -- device_idle
is no longer a flag, but an enum of (off, light, full), and there
is no information about time spent in light modes.

Also added new data about the maximum duration spent in both light
and full idle modes, to get a better understanding of how those
are behaving.

And did a little cleanup of DeviceIdleController, removing the
sensing alarm which was redundant with the regular alarm.

Change-Id: Ibeea6659577dc02deff58f048f97fcd9b0223307