Fix bug where C.TIME_UNSET was used for calcutations.

The presentationTimeOffsetMs may be C.TIME_UNSET for VOD content
and shouldn't be used in calculations for the windowStartTime.

PiperOrigin-RevId: 285363095

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DashMediaSource.java

@@ -1011,8 +1011,13 @@ public final class DashMediaSource extends BaseMediaSource {
             windowDurationUs / 2);
       }
     }
-    long windowStartTimeMs = manifest.availabilityStartTimeMs
-        + manifest.getPeriod(0).startMs + C.usToMs(currentStartTimeUs);
+    long windowStartTimeMs = C.TIME_UNSET;
+    if (manifest.availabilityStartTimeMs != C.TIME_UNSET) {
+      windowStartTimeMs =
+          manifest.availabilityStartTimeMs
+              + manifest.getPeriod(0).startMs
+              + C.usToMs(currentStartTimeUs);
+    }
     DashTimeline timeline =
         new DashTimeline(
             manifest.availabilityStartTimeMs,