Tune delivery and batching of alarms

* Inexact alarms no longer coalesce with exact alarms.  The motivation here
is that exact alarms are far more likely to be wall-clock aligned, and in
general pulling all alarms toward wall-clock alignment is a bad idea.

* Wakeup times are now fuzzed within the target batch's allowed window
rather than being hard pinned at the start of the window.

Bug 18631821

Change-Id: Iefaf34eee3f2a6546abefc27e177ee2fdcff935f