Stop audio when leaving conversation.

Fixes #4290
// FREEBIE

src/org/thoughtcrime/securesms/ConversationActivity.java

@@ -61,6 +61,7 @@ import com.google.protobuf.ByteString;
 import org.thoughtcrime.redphone.RedPhone;
 import org.thoughtcrime.redphone.RedPhoneService;
 import org.thoughtcrime.securesms.TransportOptions.OnTransportChangedListener;
+import org.thoughtcrime.securesms.audio.AudioSlidePlayer;
 import org.thoughtcrime.securesms.color.MaterialColor;
 import org.thoughtcrime.securesms.components.AnimatingToggle;
 import org.thoughtcrime.securesms.components.ComposeText;
@@ -280,6 +281,7 @@ public class ConversationActivity extends PassphraseRequiredActionBarActivity
     MessageNotifier.setVisibleThread(-1L);
     if (isFinishing()) overridePendingTransition(R.anim.fade_scale_in, R.anim.slide_to_right);
     quickAttachmentDrawer.onPause();
+    AudioSlidePlayer.stopAll();
   }
 
   @Override

src/org/thoughtcrime/securesms/audio/AudioSlidePlayer.java

@@ -133,6 +133,12 @@ public class AudioSlidePlayer {
     this.audioAttachmentServer = null;
   }
 
+  public synchronized static void stopAll() {
+    if (playing.isPresent()) {
+      playing.get().stop();
+    }
+  }
+
   public void setListener(@NonNull Listener listener) {
     this.listener = new WeakReference<>(listener);