Use asynchronous messages for input method events.

Improves the throughput of IME event handling by ensuring that
input events do not get serialized behind UI traversal and
drawing messages such as when the UI is animating.

Added support for creating an asynchronous Handler as part of a
HandlerCaller.  It turns out we should be using an asynchronous
Handler not only in IME dispatch but also in accessibility and
wallpaper events where HandlerCaller is used.  So fixed those
services to also use an asynchronous Handler.

Change-Id: I0b19140c9d5ca6ee300c1a150c48312fd55ed8eb