Preserve window sizes when rebatching alarms

The existing code wasn't retaining the requested window bounds, if any,
and so could wind up rebatching alarms into much longer potential
delivery windows than originally demanded by the caller.  This could
wind up delivering alarms outside their designated windows entirely.

Bug 11324357

Change-Id: I4d418cd08702e397b3c7692b412d4bf51d5d9e4b