Fix issue with keeping window sequence number after repeated seeks.

The number is shelved in calls to queue.clear() to keep it for the next
media period. However, the queue may also become empty by repeated calls to
advancePlayingPeriod which may happen when seeking to an unprepared period.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=205376036

library/core/src/main/java/com/google/android/exoplayer2/MediaPeriodQueue.java

@@ -228,11 +228,13 @@ import com.google.android.exoplayer2.util.Assertions;
         reading = playing.next;
       }
       playing.release();
-      playing = playing.next;
       length--;
       if (length == 0) {
         loading = null;
+        oldFrontPeriodUid = playing.uid;
+        oldFrontPeriodWindowSequenceNumber = playing.info.id.windowSequenceNumber;
       }
+      playing = playing.next;
     } else {
       playing = loading;
       reading = loading;

library/core/src/test/java/com/google/android/exoplayer2/ExoPlayerTest.java

@@ -1981,6 +1981,44 @@ public final class ExoPlayerTest {
   }
 
   @Test
+  public void testRepeatedSeeksToUnpreparedPeriodInSameWindowKeepsWindowSequenceNumber()
+      throws Exception {
+    Timeline timeline =
+        new FakeTimeline(
+            new TimelineWindowDefinition(
+                /* periodCount= */ 2,
+                /* id= */ 0,
+                /* isSeekable= */ true,
+                /* isDynamic= */ false,
+                /* durationUs= */ 10 * C.MICROS_PER_SECOND));
+    FakeMediaSource mediaSource = new FakeMediaSource(timeline, /* manifest= */ null);
+    ActionSchedule actionSchedule =
+        new ActionSchedule.Builder("testSeekToUnpreparedPeriod")
+            .pause()
+            .waitForPlaybackState(Player.STATE_READY)
+            .seek(/* windowIndex= */ 0, /* positionMs= */ 9999)
+            .seek(/* windowIndex= */ 0, /* positionMs= */ 1)
+            .seek(/* windowIndex= */ 0, /* positionMs= */ 9999)
+            .play()
+            .build();
+    ExoPlayerTestRunner testRunner =
+        new ExoPlayerTestRunner.Builder()
+            .setMediaSource(mediaSource)
+            .setActionSchedule(actionSchedule)
+            .build()
+            .start()
+            .blockUntilEnded(TIMEOUT_MS);
+
+    testRunner.assertPlayedPeriodIndices(0, 1, 0, 1);
+    assertThat(mediaSource.getCreatedMediaPeriods())
+        .containsAllOf(
+            new MediaPeriodId(/* periodIndex= */ 0, /* windowSequenceNumber= */ 0),
+            new MediaPeriodId(/* periodIndex= */ 1, /* windowSequenceNumber= */ 0));
+    assertThat(mediaSource.getCreatedMediaPeriods())
+        .doesNotContain(new MediaPeriodId(/* periodIndex= */ 1, /* windowSequenceNumber= */ 1));
+  }
+
+  @Test
   public void testRecursivePlayerChangesReportConsistentValuesForAllListeners() throws Exception {
     // We add two listeners to the player. The first stops the player as soon as it's ready and both
     // record the state change events they receive.