Add a test for preroll ad playback with ImaAdsLoader

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=202080962

extensions/ima/build.gradle

@@ -41,9 +41,11 @@ dependencies {
     // |-- com.android.support:customtabs:26.1.0
     implementation 'com.android.support:support-v4:' + supportLibraryVersion
     implementation 'com.android.support:customtabs:' + supportLibraryVersion
+    testImplementation 'com.google.truth:truth:' + truthVersion
     testImplementation 'junit:junit:' + junitVersion
     testImplementation 'org.mockito:mockito-core:' + mockitoVersion
     testImplementation 'org.robolectric:robolectric:' + robolectricVersion
+    testImplementation project(modulePrefix + 'testutils-robolectric')
 }
 
 ext {

library/core/src/main/java/com/google/android/exoplayer2/source/ads/AdsMediaSource.java

@@ -169,8 +169,6 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
 
   }
 
-  private static final String TAG = "AdsMediaSource";
-
   private final MediaSource contentMediaSource;
   private final MediaSourceFactory adMediaSourceFactory;
   private final AdsLoader adsLoader;
@@ -338,7 +336,7 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
           Arrays.fill(adDurationsUs[adGroupIndex], oldAdCount, adCount, C.TIME_UNSET);
         }
         adGroupMediaSources[adGroupIndex][adIndexInAdGroup] = adMediaSource;
-        deferredMediaPeriodByAdMediaSource.put(adMediaSource, new ArrayList<DeferredMediaPeriod>());
+        deferredMediaPeriodByAdMediaSource.put(adMediaSource, new ArrayList<>());
         prepareChildSource(id, adMediaSource);
       }
       MediaSource mediaSource = adGroupMediaSources[adGroupIndex][adIndexInAdGroup];

library/core/src/main/java/com/google/android/exoplayer2/source/ads/SinglePeriodAdTimeline.java

@@ -15,15 +15,15 @@
  */
 package com.google.android.exoplayer2.source.ads;
 
+import android.support.annotation.VisibleForTesting;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.Timeline;
 import com.google.android.exoplayer2.source.ForwardingTimeline;
 import com.google.android.exoplayer2.util.Assertions;
 
-/**
- * A {@link Timeline} for sources that have ads.
- */
-/* package */ final class SinglePeriodAdTimeline extends ForwardingTimeline {
+/** A {@link Timeline} for sources that have ads. */
+@VisibleForTesting(otherwise = VisibleForTesting.PACKAGE_PRIVATE)
+public final class SinglePeriodAdTimeline extends ForwardingTimeline {
 
   private final AdPlaybackState adPlaybackState;