Properly reset period id and start position in ExoPlayerImpl.

This is a no-op change as the respective values are not used so far but
the change makes the current state cleaner and is less error-prone.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=217892421

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

@@ -731,20 +731,26 @@ import java.util.concurrent.CopyOnWriteArraySet;
       maskingPeriodIndex = getCurrentPeriodIndex();
       maskingWindowPositionMs = getCurrentPosition();
     }
+    MediaPeriodId mediaPeriodId =
+        resetPosition
+            ? playbackInfo.getDummyFirstMediaPeriodId(shuffleModeEnabled, window)
+            : playbackInfo.periodId;
+    long startPositionUs = resetPosition ? 0 : playbackInfo.positionUs;
+    long contentPositionUs = resetPosition ? C.TIME_UNSET : playbackInfo.contentPositionUs;
     return new PlaybackInfo(
         resetState ? Timeline.EMPTY : playbackInfo.timeline,
         resetState ? null : playbackInfo.manifest,
-        playbackInfo.periodId,
+        mediaPeriodId,
-        playbackInfo.startPositionUs,
+        startPositionUs,
-        playbackInfo.contentPositionUs,
+        contentPositionUs,
         playbackState,
         /* isLoading= */ false,
         resetState ? TrackGroupArray.EMPTY : playbackInfo.trackGroups,
         resetState ? emptyTrackSelectorResult : playbackInfo.trackSelectorResult,
-        playbackInfo.periodId,
+        mediaPeriodId,
-        playbackInfo.startPositionUs,
+        startPositionUs,
         /* totalBufferedDurationUs= */ 0,
-        playbackInfo.startPositionUs);
+        startPositionUs);
   }
 
   private void updatePlaybackInfo(

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

@@ -607,7 +607,7 @@ import java.util.Collections;
     if (resolvedSeekPosition == null) {
       // The seek position was valid for the timeline that it was performed into, but the
       // timeline has changed or is not ready and a suitable seek position could not be resolved.
-      periodId = getFirstMediaPeriodId();
+      periodId = playbackInfo.getDummyFirstMediaPeriodId(shuffleModeEnabled, window);
       periodPositionUs = C.TIME_UNSET;
       contentPositionUs = C.TIME_UNSET;
       seekPositionAdjusted = true;
@@ -762,17 +762,6 @@ import java.util.Collections;
     }
   }
 
-  private MediaPeriodId getFirstMediaPeriodId() {
-    Timeline timeline = playbackInfo.timeline;
-    if (timeline.isEmpty()) {
-      return PlaybackInfo.DUMMY_MEDIA_PERIOD_ID;
-    }
-    int firstPeriodIndex =
-        timeline.getWindow(timeline.getFirstWindowIndex(shuffleModeEnabled), window)
-            .firstPeriodIndex;
-    return new MediaPeriodId(timeline.getUidOfPeriod(firstPeriodIndex));
-  }
-
   private void resetInternal(
       boolean releaseMediaSource, boolean resetPosition, boolean resetState) {
     handler.removeMessages(MSG_DO_SOME_WORK);
@@ -801,8 +790,11 @@ import java.util.Collections;
       pendingMessages.clear();
       nextPendingMessageIndex = 0;
     }
+    MediaPeriodId mediaPeriodId =
+        resetPosition
+            ? playbackInfo.getDummyFirstMediaPeriodId(shuffleModeEnabled, window)
+            : playbackInfo.periodId;
     // Set the start position to TIME_UNSET so that a subsequent seek to 0 isn't ignored.
-    MediaPeriodId mediaPeriodId = resetPosition ? getFirstMediaPeriodId() : playbackInfo.periodId;
     long startPositionUs = resetPosition ? C.TIME_UNSET : playbackInfo.positionUs;
     long contentPositionUs = resetPosition ? C.TIME_UNSET : playbackInfo.contentPositionUs;
     playbackInfo =
@@ -1178,8 +1170,13 @@ import java.util.Collections;
           periodPosition =
               resolveSeekPosition(pendingInitialSeekPosition, /* trySubsequentPeriods= */ true);
         } catch (IllegalSeekPositionException e) {
+          MediaPeriodId firstMediaPeriodId =
+              playbackInfo.getDummyFirstMediaPeriodId(shuffleModeEnabled, window);
           playbackInfo =
-              playbackInfo.resetToNewPosition(getFirstMediaPeriodId(), C.TIME_UNSET, C.TIME_UNSET);
+              playbackInfo.resetToNewPosition(
+                  firstMediaPeriodId,
+                  /* startPositionUs= */ C.TIME_UNSET,
+                  /* contentPositionUs= */ C.TIME_UNSET);
           throw e;
         }
         pendingInitialSeekPosition = null;

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

@@ -30,7 +30,7 @@ import com.google.android.exoplayer2.trackselection.TrackSelectorResult;
    * Dummy media period id used while the timeline is empty and no period id is specified. This id
    * is used when playback infos are created with {@link #createDummy(long, TrackSelectorResult)}.
    */
-  public static final MediaPeriodId DUMMY_MEDIA_PERIOD_ID =
+  private static final MediaPeriodId DUMMY_MEDIA_PERIOD_ID =
       new MediaPeriodId(/* periodUid= */ new Object());
 
   /** The current {@link Timeline}. */
@@ -151,6 +151,24 @@ import com.google.android.exoplayer2.trackselection.TrackSelectorResult;
     this.positionUs = positionUs;
   }
 
+  /**
+   * Returns dummy media period id for the first-to-be-played period of the current timeline.
+   *
+   * @param shuffleModeEnabled Whether shuffle mode is enabled.
+   * @param window A writable {@link Timeline.Window}.
+   * @return A dummy media period id for the first-to-be-played period of the current timeline.
+   */
+  public MediaPeriodId getDummyFirstMediaPeriodId(
+      boolean shuffleModeEnabled, Timeline.Window window) {
+    if (timeline.isEmpty()) {
+      return DUMMY_MEDIA_PERIOD_ID;
+    }
+    int firstPeriodIndex =
+        timeline.getWindow(timeline.getFirstWindowIndex(shuffleModeEnabled), window)
+            .firstPeriodIndex;
+    return new MediaPeriodId(timeline.getUidOfPeriod(firstPeriodIndex));
+  }
+
   /**
    * Copies playback info and resets playing and loading position.
    *