Fix failure when a seek is performed with no enabled tracks

This issue affects ExtractorMediaSource only. We shouldn't
start loading in the case that we're prepared and have no
enabled tracks, since there's nothing that we need to load.
This was causing an assertion failure in startLoading.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=140030650

library/src/main/java/com/google/android/exoplayer2/source/ExtractorMediaPeriod.java

@@ -231,7 +231,7 @@ import java.io.IOException;
 
   @Override
   public boolean continueLoading(long playbackPositionUs) {
-    if (loadingFinished) {
+    if (loadingFinished || (prepared && enabledTrackCount == 0)) {
       return false;
     }
     boolean continuedLoading = loadCondition.open();