Make it easier to create asynchronous Handlers.

There are potentially very many Handlers owned by services
that should not be blocked by barriers introduced by UI traversals
occurring on the same thread (if that ever happens).
Add some convenience constructors to make it easy to switch
these Handlers over to being async.

Bug: 7057752
Change-Id: I64d9bffe81e7c52ada4cfad4e89d4340153f4688