Move external timeline and start position overwrites to ExoPlayerImpl.

Makes it less error-prone to accidentatly forget to set the right overwrites.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177282089

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

@@ -502,10 +502,19 @@ import java.util.concurrent.CopyOnWriteArraySet;
 
   private void handlePlaybackInfo(PlaybackInfo playbackInfo, int prepareOrStopAcks, int seekAcks,
       boolean positionDiscontinuity, @DiscontinuityReason int positionDiscontinuityReason) {
-    Assertions.checkNotNull(playbackInfo.timeline);
     pendingPrepareOrStopAcks -= prepareOrStopAcks;
     pendingSeekAcks -= seekAcks;
     if (pendingPrepareOrStopAcks == 0 && pendingSeekAcks == 0) {
+      if (playbackInfo.timeline == null) {
+        // Replace internal null timeline with externally visible empty timeline.
+        playbackInfo = playbackInfo.copyWithTimeline(Timeline.EMPTY, playbackInfo.manifest);
+      }
+      if (playbackInfo.startPositionUs == C.TIME_UNSET) {
+        // Replace internal unset start position with externally visible start position of zero.
+        playbackInfo =
+            playbackInfo.fromNewPosition(
+                playbackInfo.periodId, /* startPositionUs= */ 0, playbackInfo.contentPositionUs);
+      }
       boolean timelineOrManifestChanged = this.playbackInfo.timeline != playbackInfo.timeline
           || this.playbackInfo.manifest != playbackInfo.manifest;
       this.playbackInfo = playbackInfo;

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

@@ -632,11 +632,7 @@ import java.io.IOException;
     if (mediaSource == null || timeline == null) {
       pendingInitialSeekPosition = seekPosition;
       eventHandler
-          .obtainMessage(
-              MSG_SEEK_ACK,
-              /* seekAdjusted */ 0,
-              0,
-              timeline == null ? playbackInfo.copyWithTimeline(Timeline.EMPTY, null) : playbackInfo)
+          .obtainMessage(MSG_SEEK_ACK, /* seekAdjusted */ 0, 0, playbackInfo)
           .sendToTarget();
       return;
     }
@@ -649,10 +645,8 @@ import java.io.IOException;
       // Reset, but retain the source so that it can still be used should a seek occur.
       resetInternal(
           /* releaseMediaSource= */ false, /* resetPosition= */ true, /* resetState= */ false);
-      // Set the playback position to 0 for notifying the eventHandler (instead of C.TIME_UNSET).
-      eventHandler.obtainMessage(MSG_SEEK_ACK, /* seekAdjusted */ 1, 0,
-          playbackInfo.fromNewPosition(playbackInfo.periodId.periodIndex, /* startPositionUs= */ 0,
-              /* contentPositionUs= */ C.TIME_UNSET))
+      eventHandler
+          .obtainMessage(MSG_SEEK_ACK, /* seekAdjusted */ 1, 0, playbackInfo)
           .sendToTarget();
       return;
     }
@@ -774,21 +768,9 @@ import java.io.IOException;
   private void stopInternal(boolean reset) {
     resetInternal(
         /* releaseMediaSource= */ true, /* resetPosition= */ reset, /* resetState= */ reset);
-    PlaybackInfo publicPlaybackInfo = playbackInfo;
-    if (playbackInfo.timeline == null) {
-      // Resetting the state sets the timeline to null. Use Timeline.EMPTY for notifying the
-      // eventHandler.
-      publicPlaybackInfo = publicPlaybackInfo.copyWithTimeline(Timeline.EMPTY, null);
-    }
-    if (playbackInfo.startPositionUs == C.TIME_UNSET) {
-      // When resetting the state, set the playback position to 0 (instead of C.TIME_UNSET) for
-      // notifying the eventHandler.
-      publicPlaybackInfo =
-          publicPlaybackInfo.fromNewPosition(playbackInfo.periodId.periodIndex, 0, C.TIME_UNSET);
-    }
     int prepareOrStopAcks = pendingPrepareCount + 1;
     pendingPrepareCount = 0;
-    notifySourceInfoRefresh(prepareOrStopAcks, publicPlaybackInfo);
+    notifySourceInfoRefresh(prepareOrStopAcks, playbackInfo);
     loadControl.onStopped();
     setState(Player.STATE_IDLE);
   }
@@ -1187,9 +1169,7 @@ import java.io.IOException;
     // Reset, but retain the source so that it can still be used should a seek occur.
     resetInternal(
         /* releaseMediaSource= */ false, /* resetPosition= */ true, /* resetState= */ false);
-    // Set the playback position to 0 for notifying the eventHandler (instead of C.TIME_UNSET).
-    notifySourceInfoRefresh(prepareAcks,
-        playbackInfo.fromNewPosition(playbackInfo.periodId.periodIndex, 0, C.TIME_UNSET));
+    notifySourceInfoRefresh(prepareAcks, playbackInfo);
   }
 
   private void notifySourceInfoRefresh() {