[duplicated subtitle][YouTube] Introduce AppStreamInfo to normalize subtitles in app layer.

Introduce a new domain class `AppStreamInfo` to perform subtitle
normalization (deduplication) on the application side without modifying
the extractor data `StreamInfo`.

Previously subtitle deduplication logic modified `StreamInfo` directly,
which mixes application concerns with extractor data structures. This
change separates responsibilities by projecting extractor `StreamInfo`
into an app-level domain object.

Key points:
- Preserve original `StreamInfo` from the extractor unchanged
- Perform subtitle deduplication once when constructing `AppStreamInfo`
- Provide normalized subtitle list for player and download usage
- Ensure subtitle normalization logic is centralized and reusable
- `from()` only perform object creation
- subtitle deduplication (which requires network downloading) is in the
  loadNormalizedSubtitles().

Author: TransZAllen

app/src/main/java/org/schabi/newpipe/streams/AppStreamInfo.java

@@ -0,0 +1,109 @@
+package org.schabi.newpipe.streams;
+
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+
+import android.util.Log;
+
+import org.schabi.newpipe.extractor.stream.StreamInfo;
+import org.schabi.newpipe.extractor.stream.SubtitlesStream;
+import org.schabi.newpipe.extractor.MediaFormat;
+import org.schabi.newpipe.util.subtitle.SubtitleDeduplicator;
+import org.schabi.newpipe.util.subtitle.SubtitleOrigin;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+public final class AppStreamInfo {
+    private static final String TAG = AppStreamInfo.class.getSimpleName();
+
+    @NonNull
+    private final StreamInfo originalStreamInfo;
+    @Nullable
+    private List<SubtitlesStream> normalizedSubtitles = null;
+
+    private AppStreamInfo(@NonNull final StreamInfo original) {
+        this.originalStreamInfo = original;
+    }
+
+    // Factory method: build AppStreamInfo from raw StreamInfo.
+    @NonNull
+    public static AppStreamInfo from(@NonNull final StreamInfo info) {
+        return new AppStreamInfo(info);
+    }
+
+    @NonNull
+    public StreamInfo getOriginal() {
+        return originalStreamInfo;
+    }
+
+    @NonNull
+    public List<SubtitlesStream> loadNormalizedSubtitles() {
+        if (null == normalizedSubtitles) {
+            final List<SubtitlesStream> originalSubtitles =
+                    originalStreamInfo.getSubtitles();
+
+            normalizedSubtitles =
+                    deduplicateSubtitles(originalSubtitles);
+        }
+
+        return normalizedSubtitles;
+    }
+
+    @NonNull
+    private static List<SubtitlesStream> deduplicateSubtitles(
+            @Nullable final List<SubtitlesStream> originalSubtitles) {
+
+        if ((null == originalSubtitles) || originalSubtitles.isEmpty()) {
+            return Collections.emptyList();
+        }
+
+        final List<SubtitlesStream> newSubtitles = new ArrayList<>();
+
+        for (final SubtitlesStream oldSubtitle : originalSubtitles) {
+            final MediaFormat format = oldSubtitle.getFormat();
+            if (null == format) {
+                newSubtitles.add(oldSubtitle);
+                continue;
+            }
+
+            try {
+                final SubtitleOrigin origin =
+                        SubtitleDeduplicator.getSubtitleOrigin(
+                            oldSubtitle.isAutoGenerated(),
+                            false
+                        );
+
+                final String remoteSubtitleUrl = oldSubtitle.getContent();
+
+                final String subtitleUrl =
+                        SubtitleDeduplicator.checkAndDeduplicate(
+                                remoteSubtitleUrl,
+                                format,
+                                origin
+                        );
+
+                if (remoteSubtitleUrl.equals(subtitleUrl)) {
+                    newSubtitles.add(oldSubtitle);
+                } else {
+                    final SubtitlesStream oneNewSubtitle =
+                        new SubtitlesStream.Builder()
+                            .setContent(subtitleUrl, true)
+                            .setMediaFormat(format)
+                            .setLanguageCode(oldSubtitle.getLanguageTag())
+                            .setAutoGenerated(oldSubtitle.isAutoGenerated())
+                            .build();
+
+                    newSubtitles.add(oneNewSubtitle);
+                }
+            } catch (final Exception e) {
+                Log.w(TAG, "Subtitle deduplication failed", e);
+                newSubtitles.add(oldSubtitle);
+            }
+        }
+
+        return newSubtitles;
+    }
+
+}