Deduplicate ExtractorMediaPeriod discontinuity reporting

This change makes sure progress is being made before reporting
a discontinuity. Else in cases like having no network and
playing a live stream, we allow the discontinuity to be read
each time an internal retry occurs, meaning it gets read
repeatedly. This does no harm, but is noisy and unnecessary.

We should also not allow skipping whilst there is a pending
reset or discontinuity notification, just like we don't allow
reads.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=175953064

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

@@ -303,7 +303,8 @@ import java.util.Arrays;
 
   @Override
   public long readDiscontinuity() {
-    if (notifyDiscontinuity) {
+    if (notifyDiscontinuity
+        && (loadingFinished || getExtractedSamplesCount() > extractedSamplesCountAtStartOfLoad)) {
       notifyDiscontinuity = false;
       return lastSeekPositionUs;
     }