[roll] Roll fuchsia [session][input] Refactor: simplifies key event propagation

The old code in the input pipeline processed simultaneous key events
in a batch.  While this matched key reports from the input driver,
in practice this makes key event processing a bit more involved, while
it is only very rarely necessary, in practice almost never.

This change modifies the keyboard event to contain data of a single key
at a time.  Simultaneous keypresses are pulled apart early on in the
pipeline.  Their event timelines are decoupled as well, with
simultaneous events artificially set 1 nanosecond apart.

As result, downstream key event processing and testing become simpler.
Also, upcoming work on keymap handling will be more straightforward
when only a single key at a time is processed.

Original-Bug: 75692
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/521867
Original-Revision: 310a1a51314899d8aa790df97fc669e419c2245a
GitOrigin-RevId: ab6b27c484d205bd2a22e9e66d5d87bbae928b22
Change-Id: I605d38f8934fd8c3aecd1aa43c6ad6a1e713d5ec