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
8709ba863e