Make data sources private in SegmentDownloader

Also fix SegmentDownloader.remove to not return early

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=165295432

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DashUtil.java

@@ -111,7 +111,8 @@ public final class DashUtil {
    *
    * @param dataSource The source from which the data should be loaded.
    * @param representation The representation which initialization chunk belongs to.
-   * @return {@link ChunkIndex} of the given representation.
+   * @return The {@link ChunkIndex} of the given representation, or null if no initialization or
+   *     index data exists.
    * @throws IOException Thrown when there is an error while loading.
    * @throws InterruptedException Thrown if the thread was interrupted.
    */