Internal: b/140830852

Similar to #28310, we need to be able to synchronously take keyboard events from the platform embedder, pass them to Dart to be handled, and if they are not handled, then pass them back to the platform to give it a chance to handle them.

This is needed so that add2app has a hope of working in environments with keyboards (like desktops), and so that we can capture keys before they get to a soft keyboard, as requested way back in #5986 (and has come up repeatedly).

We need a way to determine whether key events have been handled, synchronously, so that embedders know whether to continue propagating them in the native view hierarchy.

It needs to be synchronous because key events need to be responded to synchronously on host platforms.