[input-synthesis] Make the key event synthesis rules stricter

The key event requires that each key event has a distinct timestamp. This is how
reality works.  Codify that by returning an error if this isn't the case.

*WARNING*: This will break tests that do not handle the durations
correctly.

Bug: 74682,75049
Change-Id: If1d80a70de78141ca92f372a692e3186935979e4
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/519043
Fuchsia-Auto-Submit: Filip Filmar <fmil@google.com>
Reviewed-by: Alice Neels <neelsa@google.com>
Reviewed-by: Mukesh Agrawal <quiche@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>