[roll] Roll fuchsia [magma] Add GetAsyncDispatcher() to NotificationHandler

HandleWait and HandleWaitCancel in NotificationHandler are unique to
the MSD API, so clients don't have as much documentation on aspects like
how shutdown works. They also aren't generalized to waiting for
different signals on objects, waiting for timers, or running arbitrary
code on the main thread.

Since the MSD C++ API can rely on zircon concepts, we can add a
GetAsyncDispatcher() method to the notification handler. The async
dispatcher can be used for all those circumstances.

Original-Bug: 111471
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/794670
Original-Revision: dd3f83c1b98d97110b593037e168ab2d0265b764
GitOrigin-RevId: 1030a361da78d20cd3c30d06b3de23702f47ed2d
Change-Id: I760f5b8922e1a59e83b957a4c79444e9ee139c23
diff --git a/stem b/stem
index eb9c86f..3489939 100644
--- a/stem
+++ b/stem
@@ -11,7 +11,7 @@
              githooks="integration/git-hooks"
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="307ef87b09c5b99eec303743e15f7d3f290a40fa"/>
+             revision="dd3f83c1b98d97110b593037e168ab2d0265b764"/>
   </projects>
   <hooks>
     <hook name="install-environment"