Fix issue where story video does not stop playback when app is backgrounded.

app/src/main/java/org/thoughtcrime/securesms/stories/viewer/post/StoryVideoLoader.kt

@@ -38,5 +38,6 @@ class StoryVideoLoader(
 
   override fun onPause(lifecycleOwner: LifecycleOwner) {
     callback.getVideoControlsDelegate()?.detachPlayer()
+    videoPlayer.pause()
   }
 }