fix: getPresentationStartTimeAsDate() should return time unaffected…

… by clock drift (#6790)

For our Seek To Date feature, we need the presentation start date
directly from the manifest since all of the client’s key play times are
based directly on the `availabilityStartTime` from the manifest
directly.

lib/media/presentation_timeline.js

@@ -98,7 +98,7 @@ shaka.media.PresentationTimeline = class {
     this.startTimeLocked_ = false;
 
     /** @private {?number} */
-    this.initialProgramDateTime_ = null;
+    this.initialProgramDateTime_ = presentationStartTime;
   }