Delete temporary test folders on test exit.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=138860196

library/src/androidTest/java/com/google/android/exoplayer2/upstream/cache/CachedContentIndexTest.java

@@ -40,6 +40,11 @@ public class CachedContentIndexTest extends InstrumentationTestCase {
     index = new CachedContentIndex(cacheDir);
   }
 
+  @Override
+  protected void tearDown() throws Exception {
+    TestUtil.recursiveDelete(cacheDir);
+  }
+
   public void testAddGetRemove() throws Exception {
     final String key1 = "key1";
     final String key2 = "key2";