Add ExoPlayerSettingsFragment and move playback load interval size setting into it

This fragment has been added into SettingsResourceRegistry, to allow searches
in its options.

It has been placed at the place of the previous playback load interval size
setting (so in Video and Audio settings).

app/src/main/java/org/schabi/newpipe/settings/ExoPlayerSettingsFragment.java

@@ -0,0 +1,14 @@
+package org.schabi.newpipe.settings;
+
+import android.os.Bundle;
+
+import androidx.annotation.Nullable;
+
+public class ExoPlayerSettingsFragment extends BasePreferenceFragment {
+
+    @Override
+    public void onCreatePreferences(@Nullable final Bundle savedInstanceState,
+                                    @Nullable final String rootKey) {
+        addPreferencesFromResourceRegistry();
+    }
+}

app/src/main/java/org/schabi/newpipe/settings/SettingsResourceRegistry.java

@@ -40,6 +40,7 @@ public final class SettingsResourceRegistry {
         add(PlayerNotificationSettingsFragment.class, R.xml.player_notification_settings);
         add(UpdateSettingsFragment.class, R.xml.update_settings);
         add(VideoAudioSettingsFragment.class, R.xml.video_audio_settings);
+        add(ExoPlayerSettingsFragment.class, R.xml.exoplayer_settings);
     }
 
     private SettingRegistryEntry add(