Make CacheDataSourceFactory createDataSource return specific type

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=149533820

library/src/main/java/com/google/android/exoplayer2/upstream/cache/CacheDataSourceFactory.java

@@ -65,7 +65,7 @@ public final class CacheDataSourceFactory implements DataSource.Factory {
   }
 
   @Override
-  public DataSource createDataSource() {
+  public CacheDataSource createDataSource() {
     return new CacheDataSource(cache, upstreamFactory.createDataSource(),
         cacheReadDataSourceFactory.createDataSource(),
         cacheWriteDataSinkFactory.createDataSink(), flags, eventListener);