Add factory methods for async Handlers

Message.setAsynchronous is already public and this makes working
with posted messages that do not interact with UIs quite a bit nicer.

Added as factory methods for better integration with support shims
as well as generally encouraging the use of the handler callback
instead of subclassing for working with custom messages.

Bug: 73124158

Change-Id: I1210e7dc524fdf771e7af19297d5a822fdbd502c