Update Mpd parser duration calculation.

Set the duration to the sum of the final period's start
+ duration (if available) if MPD@mediaPresentationDuration
isn't set in the manifest.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117141391

library/src/main/java/com/google/android/exoplayer/dash/mpd/MediaPresentationDescriptionParser.java

@@ -149,12 +149,11 @@ public class MediaPresentationDescriptionParser extends DefaultHandler
       }
     } while (!ParserUtil.isEndTag(xpp, "MPD"));
 
-    if (!dynamic && durationMs == -1) {
-      // The manifest is static and doesn't define a duration. This is unexpected.
+    if (durationMs == -1) {
       if (nextPeriodStartMs != -1) {
         // If we know the end time of the final period, we can use it as the duration.
         durationMs = nextPeriodStartMs;
-      } else {
+      } else if (!dynamic) {
         throw new ParserException("Unable to determine duration of static manifest.");
       }
     }