Fix a bunch of position problems.

There are a couple of problems with how positions in PlaybackInfo are set at
the moment:
1. PositionUs isn't allowed to be C.TIME_UNSET. This is prevented by always
resolving to the current default position if needed.
2. In some places a window position was used as a period position. Use correct
position resolution procedure.
3. When creating a placeholder position to restart playback, we used the first
period in a window, not the one where the default position is.
4. The start position for ads was in some cases set to 0 without checking
the ad resume position.

PiperOrigin-RevId: 290749042

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

@@ -144,7 +144,7 @@ import java.util.concurrent.TimeoutException;
             ExoPlayerImpl.this.handleEvent(msg);
           }
         };
-    playbackInfo = PlaybackInfo.createDummy(/* startPositionUs= */ 0, emptyTrackSelectorResult);
+    playbackInfo = PlaybackInfo.createDummy(emptyTrackSelectorResult);
     pendingListenerNotifications = new ArrayDeque<>();
     if (analyticsCollector != null) {
       analyticsCollector.setPlayer(this);
@@ -795,17 +795,6 @@ import java.util.concurrent.TimeoutException;
       @DiscontinuityReason int positionDiscontinuityReason) {
     pendingOperationAcks -= operationAcks;
     if (pendingOperationAcks == 0) {
-      if (playbackInfo.startPositionUs == C.TIME_UNSET) {
-        // Replace internal unset start position with externally visible start position of zero.
-        playbackInfo =
-            playbackInfo.copyWithNewPosition(
-                playbackInfo.periodId,
-                /* positionUs= */ 0,
-                playbackInfo.contentPositionUs,
-                playbackInfo.totalBufferedDurationUs,
-                playbackInfo.trackGroups,
-                playbackInfo.trackSelectorResult);
-      }
       if (!this.playbackInfo.timeline.isEmpty() && playbackInfo.timeline.isEmpty()) {
         // Update the masking variables, which are used when the timeline becomes empty.
         resetMaskingPosition();
@@ -841,7 +830,7 @@ import java.util.concurrent.TimeoutException;
       timeline = Timeline.EMPTY;
       mediaPeriodId = PlaybackInfo.getDummyPeriodForEmptyTimeline();
       contentPositionUs = C.TIME_UNSET;
-      startPositionUs = C.TIME_UNSET;
+      startPositionUs = 0;
     }
     return new PlaybackInfo(
         timeline,

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

@@ -28,7 +28,7 @@ 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)}.
+   * is used when playback infos are created with {@link #createDummy(TrackSelectorResult)}.
    */
   private static final MediaPeriodId DUMMY_MEDIA_PERIOD_ID =
       new MediaPeriodId(/* periodUid= */ new Object());
@@ -83,17 +83,15 @@ import com.google.android.exoplayer2.trackselection.TrackSelectorResult;
    * Creates empty dummy playback info which can be used for masking as long as no real playback
    * info is available.
    *
-   * @param startPositionUs The start position at which playback should start, in microseconds.
    * @param emptyTrackSelectorResult An empty track selector result with null entries for each
    *     renderer.
    * @return A dummy playback info.
    */
-  public static PlaybackInfo createDummy(
-      long startPositionUs, TrackSelectorResult emptyTrackSelectorResult) {
+  public static PlaybackInfo createDummy(TrackSelectorResult emptyTrackSelectorResult) {
     return new PlaybackInfo(
         Timeline.EMPTY,
         DUMMY_MEDIA_PERIOD_ID,
-        startPositionUs,
+        /* startPositionUs= */ 0,
         /* contentPositionUs= */ C.TIME_UNSET,
         Player.STATE_IDLE,
         /* playbackError= */ null,
@@ -101,9 +99,9 @@ import com.google.android.exoplayer2.trackselection.TrackSelectorResult;
         TrackGroupArray.EMPTY,
         emptyTrackSelectorResult,
         DUMMY_MEDIA_PERIOD_ID,
-        startPositionUs,
+        /* bufferedPositionUs= */ 0,
         /* totalBufferedDurationUs= */ 0,
-        startPositionUs);
+        /* positionUs= */ 0);
   }
 
   /**