This commit fixes two issues:
1. The state maintained by `fixed` keeps the schedule's start time and
   the last run. The last run was wrongly initialized to the start time
   instead of `start + interval`. This caused a redundant immediate
   repetition when starting.

2. The delay computation was wrongly computed as `(now - start) %
   interval`. For example, given `start = 0, now = 5000,
   intervalMillis = 3000`, the delay would be `2000` instead of
   `1000`.

   This is fixed by computing the delay as `interval - ((now - start)
   % interval)`.