Add missing onLoadStarted callback to Extractor and SingleSample media period.

We added the other callbacks some time ago, but didn't include onLoadStarted.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181743764

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

@@ -168,6 +168,7 @@ import java.util.Arrays;
     sampleQueues = new SampleQueue[0];
     sampleQueues = new SampleQueue[0];
     pendingResetPositionUs = C.TIME_UNSET;
     pendingResetPositionUs = C.TIME_UNSET;
     length = C.LENGTH_UNSET;
     length = C.LENGTH_UNSET;
+    durationUs = C.TIME_UNSET;
     // Assume on-demand for MIN_RETRY_COUNT_DEFAULT_FOR_MEDIA, until prepared.
     // Assume on-demand for MIN_RETRY_COUNT_DEFAULT_FOR_MEDIA, until prepared.
     actualMinLoadableRetryCount =
     actualMinLoadableRetryCount =
         minLoadableRetryCount == ExtractorMediaSource.MIN_RETRY_COUNT_DEFAULT_FOR_MEDIA
         minLoadableRetryCount == ExtractorMediaSource.MIN_RETRY_COUNT_DEFAULT_FOR_MEDIA
@@ -436,7 +437,7 @@ import java.util.Arrays;
         /* trackFormat= */ null,
         /* trackFormat= */ null,
         C.SELECTION_REASON_UNKNOWN,
         C.SELECTION_REASON_UNKNOWN,
         /* trackSelectionData= */ null,
         /* trackSelectionData= */ null,
-        /* mediaStartTimeUs= */ 0,
+        /* mediaStartTimeUs= */ loadable.seekTimeUs,
         durationUs,
         durationUs,
         elapsedRealtimeMs,
         elapsedRealtimeMs,
         loadDurationMs,
         loadDurationMs,
@@ -456,7 +457,7 @@ import java.util.Arrays;
         /* trackFormat= */ null,
         /* trackFormat= */ null,
         C.SELECTION_REASON_UNKNOWN,
         C.SELECTION_REASON_UNKNOWN,
         /* trackSelectionData= */ null,
         /* trackSelectionData= */ null,
-        /* mediaStartTimeUs= */ 0,
+        /* mediaStartTimeUs= */ loadable.seekTimeUs,
         durationUs,
         durationUs,
         elapsedRealtimeMs,
         elapsedRealtimeMs,
         loadDurationMs,
         loadDurationMs,
@@ -483,7 +484,7 @@ import java.util.Arrays;
         /* trackFormat= */ null,
         /* trackFormat= */ null,
         C.SELECTION_REASON_UNKNOWN,
         C.SELECTION_REASON_UNKNOWN,
         /* trackSelectionData= */ null,
         /* trackSelectionData= */ null,
-        /* mediaStartTimeUs= */ 0,
+        /* mediaStartTimeUs= */ loadable.seekTimeUs,
         durationUs,
         durationUs,
         elapsedRealtimeMs,
         elapsedRealtimeMs,
         loadDurationMs,
         loadDurationMs,
@@ -595,7 +596,17 @@ import java.util.Arrays;
       pendingResetPositionUs = C.TIME_UNSET;
       pendingResetPositionUs = C.TIME_UNSET;
     }
     }
     extractedSamplesCountAtStartOfLoad = getExtractedSamplesCount();
     extractedSamplesCountAtStartOfLoad = getExtractedSamplesCount();
-    loader.startLoading(loadable, this, actualMinLoadableRetryCount);
+    long elapsedRealtimeMs = loader.startLoading(loadable, this, actualMinLoadableRetryCount);
+    eventDispatcher.loadStarted(
+        loadable.dataSpec,
+        C.DATA_TYPE_MEDIA,
+        C.TRACK_TYPE_UNKNOWN,
+        /* trackFormat= */ null,
+        C.SELECTION_REASON_UNKNOWN,
+        /* trackSelectionData= */ null,
+        /* mediaStartTimeUs= */ loadable.seekTimeUs,
+        durationUs,
+        elapsedRealtimeMs);
   }
   }
 
 
   private void configureRetry(ExtractingLoadable loadable) {
   private void configureRetry(ExtractingLoadable loadable) {

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

@@ -44,7 +44,7 @@ public interface MediaSourceEventListener {
    * @param mediaStartTimeMs The start time of the media being loaded, or {@link C#TIME_UNSET} if
    * @param mediaStartTimeMs The start time of the media being loaded, or {@link C#TIME_UNSET} if
    *     the load is not for media data.
    *     the load is not for media data.
    * @param mediaEndTimeMs The end time of the media being loaded, or {@link C#TIME_UNSET} if the
    * @param mediaEndTimeMs The end time of the media being loaded, or {@link C#TIME_UNSET} if the
-   *     load is not for media data.
+   *     load is not for media data or the end time is unknown.
    * @param elapsedRealtimeMs The value of {@link SystemClock#elapsedRealtime} when the load began.
    * @param elapsedRealtimeMs The value of {@link SystemClock#elapsedRealtime} when the load began.
    */
    */
   void onLoadStarted(
   void onLoadStarted(

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

@@ -133,10 +133,21 @@ import java.util.Arrays;
     if (loadingFinished || loader.isLoading()) {
     if (loadingFinished || loader.isLoading()) {
       return false;
       return false;
     }
     }
+    long elapsedRealtimeMs =
         loader.startLoading(
         loader.startLoading(
             new SourceLoadable(dataSpec, dataSourceFactory.createDataSource()),
             new SourceLoadable(dataSpec, dataSourceFactory.createDataSource()),
             this,
             this,
             minLoadableRetryCount);
             minLoadableRetryCount);
+    eventDispatcher.loadStarted(
+        dataSpec,
+        C.DATA_TYPE_MEDIA,
+        C.TRACK_TYPE_UNKNOWN,
+        format,
+        C.SELECTION_REASON_UNKNOWN,
+        /* trackSelectionData= */ null,
+        /* mediaStartTimeUs= */ 0,
+        durationUs,
+        elapsedRealtimeMs);
     return true;
     return true;
   }
   }