Announce a session's end when it is actually over, and for closes too

The phone's notification kept saying '1 shell connected' after the
shell was gone, and both close routes were at fault. A shell exiting on
its own raised SessionEnded from inside its run's finally block — where
the run task is by definition not yet complete, so the LiveSessionCount
the keep-alive reads still counted the dead shell, and nothing fired
later to correct it. A tab closed by hand announced nothing at all, by
a recorded decision that assumed every subscriber was the closer; the
keep-alive is not, and a close it never heard about left the
notification claiming a shell over nothing.

The end is now announced from a continuation after the run completes,
and CloseSessionAsync announces after its own drain — every subscriber
was already a reconcile-to-reality handler, so the echo the old remark
feared costs nothing. Shutdown stays silent: it is dismantling the
subscribers along with the sessions.

src/DodoSSH.Client.Android/Platform/SessionKeepAlive.cs

@@ -46,8 +46,11 @@ internal sealed class SessionKeepAlive : IDisposable
         this.activeTransfers = activeTransfers;
         this.holdsFileSession = holdsFileSession;
 
-        // Raised on whatever thread the pump unwound on, which is fine: starting and stopping a service is
-        // a binder call and needs no particular thread. Nothing here touches the interface.
+        // Raised on whatever thread the workspace announced from — a continuation of the ended run, or the
+        // closer's own — which is fine: starting and stopping a service is a binder call and needs no
+        // particular thread. Nothing here touches the interface. That the announcement waits for the run to
+        // actually complete, and comes for deliberate closes too, is what makes reading LiveSessionCount
+        // from it honest — the event's own remark carries the stuck notification that taught us both.
         workspace.SessionEnded += OnSessionEnded;
     }