Improve ExoCacheTest.

- Use FakeDataSource as the upstream source.
- Actually validate that caching is happening (i.e. reads happen
  on the upstream source only if the data hasn't been read through
  the CacheDataSource already).
- Move FakeClock to sit alongside the other Fake classes.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=118555903

library/src/androidTest/java/com/google/android/exoplayer/util/FakeClock.java → library/src/androidTest/java/com/google/android/exoplayer/testutil/FakeClock.java

@@ -13,7 +13,9 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-package com.google.android.exoplayer.util;
+package com.google.android.exoplayer.testutil;
+
+import com.google.android.exoplayer.util.Clock;
 
 /**
  * A {@link Clock} that returns a fixed value specified in the constructor.

library/src/androidTest/java/com/google/android/exoplayer/testutil/FakeDataSource.java

@@ -42,6 +42,7 @@ import java.util.ArrayList;
 public final class FakeDataSource implements DataSource {
 
   private final ArrayList<Segment> segments;
+  private final ArrayList<DataSpec> openedDataSpecs;
 
   private final boolean simulateUnknownLength;
   private final long totalLength;
@@ -59,6 +60,7 @@ public final class FakeDataSource implements DataSource {
       totalLength += segment.length;
     }
     this.totalLength = totalLength;
+    openedDataSpecs = new ArrayList<DataSpec>();
   }
 
   @Override
@@ -67,11 +69,12 @@ public final class FakeDataSource implements DataSource {
     // DataSpec requires a matching close call even if open fails.
     opened = true;
     uri = dataSpec.uri;
+    openedDataSpecs.add(dataSpec);
     // If the source knows that the request is unsatisfiable then fail.
     if (dataSpec.position >= totalLength) {
       throw new IOException("Unsatisfiable position");
     } else if (dataSpec.length != C.LENGTH_UNBOUNDED
-        && dataSpec.position + dataSpec.length >= totalLength) {
+        && dataSpec.position + dataSpec.length > totalLength) {
       throw new IOException("Unsatisfiable range");
     }
     // Scan through the segments, configuring them for the current read.
@@ -148,6 +151,17 @@ public final class FakeDataSource implements DataSource {
     }
   }
 
+  /**
+   * @return The {@link DataSpec} instances passed to {@link #open(DataSpec)} since the last call
+   *     to this method.
+   */
+  public DataSpec[] getAndClearOpenedDataSpecs() {
+    DataSpec[] dataSpecs = new DataSpec[openedDataSpecs.size()];
+    openedDataSpecs.toArray(dataSpecs);
+    openedDataSpecs.clear();
+    return dataSpecs;
+  }
+
   private static class Segment {
 
     public final IOException exception;