adb: change handler only when recognized

ADB devices must take new handler into account only when they recognize it.
This lets operating systems probe for valid/invalid handles, to know device capabilities.

Add a FIXME in keyboard handler, which should use a different translation
table depending of the selected handler.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
