Enhance DefaultTrackSelector part 2

- Support specifying a preferred text language.
- Score based selection for text/audio/other tracks.
- Prefer default tracks to non-default ones.
- Allow overriding of base select*Track methods.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=129626994

library/src/main/java/com/google/android/exoplayer2/util/Util.java

@@ -240,6 +240,16 @@ public final class Util {
   }
 
   /**
+   * Returns a normalized RFC 5646 language code.
+   *
+   * @param language A possibly non-normalized RFC 5646 language code.
+   * @return The normalized code, or null if the input was null.
+   */
+  public static String normalizeLanguageCode(String language) {
+    return language == null ? null : new Locale(language).getLanguage();
+  }
+
+  /**
    * Returns a new byte array containing the code points of a {@link String} encoded using UTF-8.
    *
    * @param value The {@link String} whose bytes should be obtained.