[roll] Roll fuchsia [ui][input-pipeline] use async_enter for handle_input_event()

When tracing calls to `handle_input_event()`, use `async_enter!()`
instead of `async_duration!()`.

If we use `async_duration!()`, Perfetto renders the slice on
the track for Scene Manager's initial thread. While that makes
sense for code that's CPU bound, it can be confusing for code
that blocks on I/O.

If we use `async_enter!()`, Perfetto renders the slice on its
own track, within the group of tracks for Scene Manager.

The screenshots in PS1 illustrate the difference. Note that
I'll remove the screenshots in PS2.

While there:
- Adjust the `handle_input_event()` trace events to exclude
  overhead in the `InputPipelineAssembly`. Specifically, the
  time required to propagate input events to the next handler
  is no longer counted as part of a handler's cost.
- Remove stale comment about a log message. A previous commit
  replaced the handler index with the handler name; the log
  message is now appropriately specific.

Original-Bug: None
Test: manual (see screenshot)
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/744870
Original-Revision: e5b9a01e2d5d98cca5995a52dff7346af3eb0f71
GitOrigin-RevId: 90f98133614d94eecfe3339b78d2e9e3d96e446f
Change-Id: Ia576ff6290f8803d8ef1766e05b6194bce7034c0