Make text information frame values abstract

Make the values of TextInformationFrame an abstract list instead of an
explicit ImmutableList.

library/extractor/src/main/java/com/google/android/exoplayer2/metadata/id3/TextInformationFrame.java

@@ -37,7 +37,7 @@ public final class TextInformationFrame extends Id3Frame {
   public final String value;
   public final String value;
 
 
   @NonNull
   @NonNull
-  public final ImmutableList<String> values;
+  public final List<String> values;
 
 
   public TextInformationFrame(String id, @Nullable String description, @NonNull List<String> values) {
   public TextInformationFrame(String id, @Nullable String description, @NonNull List<String> values) {
     super(id);
     super(id);