backends/native: Set up keyboard a11y when keyboards are plugged

Commit 4e0ffba5 attempted to fix initialization of keyboard a11y, but mousekeys do attempt to create a virtual input device at a time that it is too early to try to create one.

Defer this operation until keyboard devices are added, so that we are ensured to already have the seat input thread set up.

Fixes: 4e0ffba5 - backends/native: Initialize keyboard a11y on startup