fix(ringtones): let a sound preview be stopped

The picker's play button only ever started a preview; nothing but taking
a row, playing another one or leaving the picker stopped it. The
previewer now publishes what it is sounding, cleared on stop and when the
tone ends, and the playing row's button becomes a stop button.

app/src/test/java/de/jeanlucmakiola/clockula/testing/FakeRingtones.kt

@@ -4,6 +4,7 @@ import de.jeanlucmakiola.clockula.data.ringtones.RingtoneCatalog
 import de.jeanlucmakiola.clockula.data.ringtones.RingtonePreviewer
 import de.jeanlucmakiola.clockula.domain.RingtoneOption
 import kotlinx.coroutines.CompletableDeferred
+import kotlinx.coroutines.flow.MutableStateFlow
 
 /**
  * The device's sound catalog, scripted: which tones it has, what it can name,
@@ -68,11 +69,15 @@ class FakeRingtonePreviewer : RingtonePreviewer {
 
     val events: MutableList<PreviewEvent> = mutableListOf()
 
+    override val playing = MutableStateFlow<String?>(null)
+
     override suspend fun play(uri: String) {
         events += PreviewEvent.Played(uri)
+        playing.value = uri
     }
 
     override fun stop() {
         events += PreviewEvent.Stopped
+        playing.value = null
     }
 }