Normalize language code independently of device language

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=184120873

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

@@ -262,7 +262,7 @@ public final class Util {
     try {
       return language == null ? null : new Locale(language).getISO3Language();
     } catch (MissingResourceException e) {
-      return language.toLowerCase();
+      return toLowerInvariant(language);
     }
   }