Work on issue #20401752: device idle and alarms.

Main change is to fix problems in the alarm manager where it
was not doing a good job at scheduling the device idle wakeup
before an alarm.  To fix this, we now keep explicit track of
the next "wake from idle" alarm so we can consistently use that
to adjust the time of the device idle alarm if needed.

Also a bunch of debugging stuff to help further diagnose this
problem:

- Alarm summary now includes the last time the alarm went off.
- Improved dumpsys output of alarms.
- New event log tags for changes related to idle state.

Change-Id: Idd2c8d24ae23379f7c75815ddbc6835c2a04656c