Fix an issue with seeking that can lead to STATE_END not delivered.

GitHub: #1897

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=174841175

library/core/src/main/java/com/google/android/exoplayer2/source/CompositeSequenceableLoader.java

@@ -63,7 +63,11 @@ public final class CompositeSequenceableLoader implements SequenceableLoader {
         break;
       }
       for (SequenceableLoader loader : loaders) {
-        if (loader.getNextLoadPositionUs() == nextLoadPositionUs) {
+        long loaderNextLoadPositionUs = loader.getNextLoadPositionUs();
+        boolean isLoaderBehind =
+            loaderNextLoadPositionUs != C.TIME_END_OF_SOURCE
+                && loaderNextLoadPositionUs <= positionUs;
+        if (loaderNextLoadPositionUs == nextLoadPositionUs || isLoaderBehind) {
           madeProgressThisIteration |= loader.continueLoading(positionUs);
         }
       }