Fix NPE in FakeAdaptiveMediaPeriod.

The compositeSequenableLoader was causing NPEs in isLoading. Initializing it
upfront prevents this problem and is in line with what we do in all real
MediaPeriods.

PiperOrigin-RevId: 275491511

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeAdaptiveMediaPeriod.java

@@ -60,6 +60,7 @@ public class FakeAdaptiveMediaPeriod extends FakeMediaPeriod
     this.transferListener = transferListener;
     this.durationUs = durationUs;
     this.sampleStreams = newSampleStreamArray(0);
+    this.sequenceableLoader = new CompositeSequenceableLoader(new SequenceableLoader[0]);
   }
 
   @Override