Added Mp3ExtractorTest and modified Mp3Extractor to pass the test.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=123640738

library/src/androidTest/java/com/google/android/exoplayer/extractor/mp3/Mp3ExtractorTest.java

+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.android.exoplayer.extractor.mp3;
+
+import com.google.android.exoplayer.extractor.Extractor;
+import com.google.android.exoplayer.testutil.TestUtil;
+
+import android.test.InstrumentationTestCase;
+
+/**
+ * Unit test for {@link Mp3Extractor}.
+ */
+public final class Mp3ExtractorTest extends InstrumentationTestCase {
+
+  public void testMp3Sample() throws Exception {
+    TestUtil.assertOutput(new TestUtil.ExtractorFactory() {
+      @Override
+      public Extractor create() {
+        return new Mp3Extractor();
+      }
+    }, "mp3/bear.mp3", getInstrumentation());
+  }
+
+}

library/src/main/java/com/google/android/exoplayer/extractor/mp3/Mp3Extractor.java

@@ -123,7 +123,7 @@ public final class Mp3Extractor implements Extractor {
       return RESULT_END_OF_INPUT;
     }
     if (seeker == null) {
-      setupSeeker(input);
+      seeker = setupSeeker(input);
       extractorOutput.seekMap(seeker);
       trackOutput.format(Format.createAudioSampleFormat(null, synchronizedHeader.mimeType,
           Format.NO_VALUE, MpegAudioHeader.MAX_FRAME_SIZE_BYTES, synchronizedHeader.channels,
@@ -263,17 +263,18 @@ public final class Mp3Extractor implements Extractor {
   }
 
   /**
-   * Sets {@link #seeker} to seek using metadata read from {@code input}, which should provide data
-   * from the start of the first frame in the stream. On returning, the input's position will be set
-   * to the start of the first frame of audio.
+   * Returns a {@link Seeker} to seek using metadata read from {@code input}, which should provide
+   * data from the start of the first frame in the stream. On returning, the input's position will
+   * be set to the start of the first frame of audio.
    *
    * @param input The {@link ExtractorInput} from which to read.
    * @throws IOException Thrown if there was an error reading from the stream. Not expected if the
    *     next two frames were already peeked during synchronization.
    * @throws InterruptedException Thrown if reading from the stream was interrupted. Not expected if
    *     the next two frames were already peeked during synchronization.
+   * @return a {@link Seeker}.
    */
-  private void setupSeeker(ExtractorInput input) throws IOException, InterruptedException {
+  private Seeker setupSeeker(ExtractorInput input) throws IOException, InterruptedException {
     // Read the first frame which may contain a Xing or VBRI header with seeking metadata.
     ParsableByteArray frame = new ParsableByteArray(synchronizedHeader.frameSize);
     input.peekFully(frame.data, 0, synchronizedHeader.frameSize);
@@ -287,6 +288,7 @@ public final class Mp3Extractor implements Extractor {
         : (synchronizedHeader.channels != 1 ? 21 : 13); // MPEG 2 or 2.5
     frame.setPosition(xingBase);
     int headerData = frame.readInt();
+    Seeker seeker = null;
     if (headerData == XING_HEADER || headerData == INFO_HEADER) {
       seeker = XingSeeker.create(synchronizedHeader, frame, position, length);
       if (seeker != null && !gaplessInfoHolder.hasGaplessInfo()) {
@@ -317,6 +319,8 @@ public final class Mp3Extractor implements Extractor {
       MpegAudioHeader.populateHeader(scratch.readInt(), synchronizedHeader);
       seeker = new ConstantBitrateSeeker(input.getPosition(), synchronizedHeader.bitrate, length);
     }
+
+    return seeker;
   }
 
   /**