Fix stuck-buffering state when playing merged media

Also added a TODO to track clarifying SequenceableLoader
more accurately and auditing existing implementations.

Issue: #2396

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=150305685

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

@@ -354,6 +354,7 @@ import java.io.IOException;
       sourceListener.onSourceInfoRefreshed(
           new SinglePeriodTimeline(durationUs, seekMap.isSeekable()), null);
     }
+    callback.onContinueLoadingRequested(this);
   }
 
   @Override

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

@@ -17,6 +17,7 @@ package com.google.android.exoplayer2.source;
 
 import com.google.android.exoplayer2.C;
 
+// TODO: Clarify the requirements for implementing this interface [Internal ref: b/36250203].
 /**
  * A loader that can proceed in approximate synchronization with other loaders.
  */