Allow defining replacement key events in keymap

Currently keyboard maps allow to assign character sequences to key
events and allow specifying a so-called "fallback" key events that are
re-injected into input stream if target application indicates that it
was not able to handle the original key event. Unfortunately there is no
way to perform substitution before handing the event to applicationis.

This change adds a new keymap keyword "replace" that allows users query
"replacement" actions for key (if any), with the intent that such
replacement happens early in the event handling process.

Bug: 24504154

Change-Id: I3e6a2476c856524171df00ad22ff56f2018c1278