Fix playback of media with >1MB preparation data

Also clarify when getNextLoadPositionUs and continueLoading
can be called.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=142124497

library/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java

@@ -676,6 +676,7 @@ import java.io.IOException;
     standaloneMediaClock.stop();
     rendererMediaClock = null;
     rendererMediaClockSource = null;
+    rendererPositionUs = RENDERER_TIMESTAMP_OFFSET_US;
     for (Renderer renderer : enabledRenderers) {
       try {
         ensureStopped(renderer);
@@ -823,9 +824,6 @@ import java.io.IOException;
   }
 
   private boolean haveSufficientBuffer(boolean rebuffering) {
-    if (loadingPeriodHolder == null) {
-      return false;
-    }
     long loadingPeriodBufferedPositionUs = !loadingPeriodHolder.prepared
         ? loadingPeriodHolder.startPositionUs
         : loadingPeriodHolder.mediaPeriod.getBufferedPositionUs();
@@ -1287,7 +1285,8 @@ import java.io.IOException;
   }
 
   private void maybeContinueLoading() {
-    long nextLoadPositionUs = loadingPeriodHolder.mediaPeriod.getNextLoadPositionUs();
+    long nextLoadPositionUs = !loadingPeriodHolder.prepared ? 0
+        : loadingPeriodHolder.mediaPeriod.getNextLoadPositionUs();
     if (nextLoadPositionUs == C.TIME_END_OF_SOURCE) {
       setIsLoading(false);
     } else {

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

@@ -244,7 +244,7 @@ import java.io.IOException;
 
   @Override
   public long getNextLoadPositionUs() {
-    return getBufferedPositionUs();
+    return enabledTrackCount == 0 ? C.TIME_END_OF_SOURCE : getBufferedPositionUs();
   }
 
   @Override

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

@@ -133,4 +133,32 @@ public interface MediaPeriod extends SequenceableLoader {
    */
   long seekToUs(long positionUs);
 
+  // SequenceableLoader interface. Overridden to provide more specific documentation.
+
+  /**
+   * Returns the next load time, or {@link C#TIME_END_OF_SOURCE} if loading has finished.
+   * <p>
+   * This method should only be called after the period has been prepared. It may be called when no
+   * tracks are selected.
+   */
+  @Override
+  long getNextLoadPositionUs();
+
+  /**
+   * Attempts to continue loading.
+   * <p>
+   * This method may be called both during and after the period has been prepared.
+   * <p>
+   * A period may call {@link Callback#onContinueLoadingRequested(SequenceableLoader)} on the
+   * {@link Callback} passed to {@link #prepare(Callback)} to request that this method be called
+   * when the period is permitted to continue loading data. A period may do this both during and
+   * after preparation.
+   *
+   * @param positionUs The current playback position.
+   * @return True if progress was made, meaning that {@link #getNextLoadPositionUs()} will return
+   *     a different value than prior to the call. False otherwise.
+   */
+  @Override
+  boolean continueLoading(long positionUs);
+
 }