[async] Change async_t -> async_dispatcher_t

Change name of dispatcher type from async_t to async_dispatcher_t
Keep typedef of async_t for compatibility

Change async_{get,set}_default() to async_{get,set}_default_dispatcher()
Again, keep compatible apis until other layers have been modified to use
new api.

Bug:ZX-2337
Test:CQ
Change-Id: If0b5d061fd114d60e23078dbb5f2a6ed9ed6f6cc

[async] Move AsyncStub to lib/async-testutils

Change-Id: Id05d08eaafc48c8778039e4c17b28186554ed90f

[async] Create standalone Post(Delayed)?Task(ForTime)? methods.

-Add "now" method to the async_t ops for accessing the 'current time'
(to be faked and controlled within a test fixture).
-Repurpose the existing standalone PostTask() function as
PostTaskForTime(),
and use that to implement PostTask() and PostDelayedTask(), with the
latter accessing the internal time of async_t argument.

BUG: ZX-1819

Change-Id: I50151ec67c5484b9d51d61c6f7a69b8408b2d245