Don't refresh DASH manifest if minUpdatePeriod is unset

We currently refresh repeatedly in this case. According to the
DASH spec omitting minUpdatePeriod indicates that the manifest
does not change, and therefore we should not refresh. I think
it might be valid to omit minUpdatePeriod in a dynamic manifest
if relying exclusively on EMSGs to trigger manifest refresh.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191420247

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

@@ -876,7 +876,9 @@ public final class DashMediaSource extends BaseMediaSource {
       }
       }
       if (manifestLoadPending) {
       if (manifestLoadPending) {
         startLoadingManifest();
         startLoadingManifest();
-      } else if (scheduleRefresh && manifest.dynamic) {
+      } else if (scheduleRefresh
+          && manifest.dynamic
+          && manifest.minUpdatePeriodMs != C.TIME_UNSET) {
         // Schedule an explicit refresh if needed.
         // Schedule an explicit refresh if needed.
         long minUpdatePeriodMs = manifest.minUpdatePeriodMs;
         long minUpdatePeriodMs = manifest.minUpdatePeriodMs;
         if (minUpdatePeriodMs == 0) {
         if (minUpdatePeriodMs == 0) {