Fix some issues with restoring focus between pages (#613)

## Description
Yet more focus issue fixes:
- Restore focus when going back to home page from a later row
- Don't jump to episode row when going back to series overview (such as
from a person page)

This PR also changes how backgrounding the app during playback works.
Now the only goes back to the previous page when the app comes back to
the foreground. This helps with focusing back on the page properly.
There's also a bit more clean up performed.

app/src/main/java/com/github/damontecres/wholphin/services/PlaybackLifecycleObserver.kt

@@ -20,6 +20,10 @@ class PlaybackLifecycleObserver
         private var wasPlaying: Boolean? = null
 
         override fun onStart(owner: LifecycleOwner) {
+            val lastDest = navigationManager.backStack.lastOrNull()
+            if (lastDest is Destination.Playback || lastDest is Destination.PlaybackList) {
+                navigationManager.goBack()
+            }
             wasPlaying = null
         }
 
@@ -40,9 +44,6 @@ class PlaybackLifecycleObserver
         }
 
         override fun onStop(owner: LifecycleOwner) {
-            if (navigationManager.backStack.lastOrNull() is Destination.Playback) {
-                navigationManager.goBack()
-            }
             themeSongPlayer.stop()
         }
     }