address review: signal transient dismissal with an event flow, not an int

An Int counter was already rejected for the now-playing launch signal; use
a broadcast Flow on the shared ContextMenuHost and collect it, so the
dismissal is an event rather than an ever-increasing number.

app/src/test/kotlin/com/subtracks/ui/LibraryOptionsTest.kt

@@ -119,7 +119,7 @@ class LibraryOptionsTest {
                     onOpenSettings = {},
                     sortOptions = sortOptionsFor(LibraryTab.Albums),
                     starredSupported = true,
-                    dismissRequests = host.dismissRequests,
+                    dismissals = host.dismissals,
                 )
             }
         }