Allow playback of unseekable fmp4 media.

This is useful to allow playback of individual segments from a
DASH stream as regular fmp4 files. These segments don't typically
contain a segment index. For playback to start, we need to invoke
seekMap with the UNSEEKABLE index. We do this if we haven't seen
a segment index when we encounter an mdat box (if one were present,
it would have been located earlier than this point).

library/src/main/java/com/google/android/exoplayer/extractor/mp4/FragmentedMp4Extractor.java

@@ -22,6 +22,7 @@
 import com.google.android.exoplayer.extractor.ExtractorInput;
 import com.google.android.exoplayer.extractor.ExtractorOutput;
 import com.google.android.exoplayer.extractor.PositionHolder;
+import com.google.android.exoplayer.extractor.SeekMap;
 import com.google.android.exoplayer.extractor.TrackOutput;
 import com.google.android.exoplayer.extractor.mp4.Atom.ContainerAtom;
 import com.google.android.exoplayer.extractor.mp4.Atom.LeafAtom;
@@ -94,6 +95,9 @@ public final class FragmentedMp4Extractor implements Extractor {
   private ExtractorOutput extractorOutput;
   private TrackOutput trackOutput;
 
+  // Whether extractorOutput.seekMap has been invoked.
+  private boolean haveOutputSeekMap;
+
   public FragmentedMp4Extractor() {
     this(0);
   }
@@ -182,6 +186,10 @@ private boolean readAtomHeader(ExtractorInput input) throws IOException, Interru
     atomType = atomHeader.readInt();
 
     if (atomType == Atom.TYPE_mdat) {
+      if (!haveOutputSeekMap) {
+        extractorOutput.seekMap(SeekMap.UNSEEKABLE);
+        haveOutputSeekMap = true;
+      }
       if (fragmentRun.sampleEncryptionDataNeedsFill) {
         parserState = STATE_READING_ENCRYPTION_DATA;
       } else {
@@ -233,6 +241,7 @@ private void onLeafAtomRead(LeafAtom leaf, long inputPosition) {
     } else if (leaf.type == Atom.TYPE_sidx) {
       ChunkIndex segmentIndex = parseSidx(leaf.data, inputPosition);
       extractorOutput.seekMap(segmentIndex);
+      haveOutputSeekMap = true;
     }
   }