Call exoPlayer.prepare() on PlaybackPreparer.onPrepare()

If a playbackPreparer is set, then instead of calling `player.prepare()`, the MediaSessionConnector will call `playbackPreparer.onPrepare(true)` instead, as seen below.
This commit makes it so that playbackPreparer.onPrepare(true) restores the original behavior of just calling player.prepare().

From MediaSessionConnector -> MediaSessionCompat.Callback implementation:
```java
    @Override
    public void onPlay() {
      if (canDispatchPlaybackAction(PlaybackStateCompat.ACTION_PLAY)) {
        if (player.getPlaybackState() == Player.STATE_IDLE) {
          if (playbackPreparer != null) {
            playbackPreparer.onPrepare(/* playWhenReady= */ true);
          } else {
            player.prepare();
          }
        } else if (player.getPlaybackState() == Player.STATE_ENDED) {
          seekTo(player, player.getCurrentMediaItemIndex(), C.TIME_UNSET);
        }
        Assertions.checkNotNull(player).play();
      }
    }
```

app/src/main/java/org/schabi/newpipe/player/Player.java

@@ -1386,6 +1386,19 @@ public final class Player implements PlaybackListener, Listener {
     public void onCues(@NonNull final CueGroup cueGroup) {
         UIs.call(playerUi -> playerUi.onCues(cueGroup.cues));
     }
+
+    /**
+     * To be called when the {@code PlaybackPreparer} set in the {@link MediaSessionConnector}
+     * receives an {@code onPrepare()} call. This function allows restoring the default behavior
+     * that would happen if there was no playback preparer set, i.e. to just call
+     * {@code player.prepare()}. You can find the default behavior in `onPlay()` inside the
+     * {@link MediaSessionConnector} file.
+     */
+    public void onPrepare() {
+        if (!exoPlayerIsNull()) {
+            simpleExoPlayer.prepare();
+        }
+    }
     //endregion
 
 

app/src/main/java/org/schabi/newpipe/player/PlayerService.java

@@ -96,7 +96,12 @@ public final class PlayerService extends MediaBrowserServiceCompat {
         mediaBrowserPlaybackPreparer = new MediaBrowserPlaybackPreparer(
                 this,
                 sessionConnector::setCustomErrorMessage,
-                () -> sessionConnector.setCustomErrorMessage(null)
+                () -> sessionConnector.setCustomErrorMessage(null),
+                (playWhenReady) -> {
+                    if (player != null) {
+                        player.onPrepare();
+                    }
+                }
         );
         sessionConnector.setPlaybackPreparer(mediaBrowserPlaybackPreparer);
 

app/src/main/java/org/schabi/newpipe/player/mediabrowser/MediaBrowserPlaybackPreparer.kt

@@ -29,6 +29,7 @@ import org.schabi.newpipe.util.ChannelTabHelper
 import org.schabi.newpipe.util.ExtractorHelper
 import org.schabi.newpipe.util.NavigationHelper
 import java.util.function.BiConsumer
+import java.util.function.Consumer
 
 /**
  * This class is used to cleanly separate the Service implementation (in
@@ -40,11 +41,15 @@ import java.util.function.BiConsumer
  * @param setMediaSessionError takes an error String and an error code from [PlaybackStateCompat],
  * calls `sessionConnector.setCustomErrorMessage(errorString, errorCode)`
  * @param clearMediaSessionError calls `sessionConnector.setCustomErrorMessage(null)`
+ * @param onPrepare takes playWhenReady, calls `player.prepare()`; this is needed because
+ * `MediaSessionConnector`'s `onPlay()` method calls this class'  [onPrepare] instead of
+ * `player.prepare()` if the playback preparer is not null, but we want the original behavior
  */
 class MediaBrowserPlaybackPreparer(
     private val context: Context,
     private val setMediaSessionError: BiConsumer<String, Int>, // error string, error code
     private val clearMediaSessionError: Runnable,
+    private val onPrepare: Consumer<Boolean>,
 ) : PlaybackPreparer {
     private val database = NewPipeDatabase.getInstance(context)
     private var disposable: Disposable? = null
@@ -59,7 +64,7 @@ class MediaBrowserPlaybackPreparer(
     }
 
     override fun onPrepare(playWhenReady: Boolean) {
-        // TODO handle onPrepare
+        onPrepare.accept(playWhenReady)
     }
 
     override fun onPrepareFromMediaId(mediaId: String, playWhenReady: Boolean, extras: Bundle?) {