Add shutdown call after each test to initialize a new sandbox env for every test.

Before this change, only the first APCT test that ran had a non-zero
SANDBOX_INIT_TIME_AVG value. With the change, all tests populate this
value since they create a new sandbox env.

Test: Running APCT tests. Results: https://paste.googleplex.com/4864291380396032
Change-Id: Ic6285a0695e7559181bf09ab4aca5e905f14a984

apct-tests/perftests/rubidium/src/android/rubidium/js/JSScriptEnginePerfTests.java

@@ -48,6 +48,7 @@ import com.google.common.collect.ImmutableList;
 import com.google.common.util.concurrent.ListenableFuture;
 
 import org.json.JSONArray;
+import org.junit.After;
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
@@ -86,6 +87,11 @@ public class JSScriptEnginePerfTests {
                 "function test() { return \"hello world\";" + " }", ImmutableList.of(), "test");
     }
 
+    @After
+    public void after() {
+        sJSScriptEngine.shutdown();
+    }
+
     @Test
     public void evaluate_helloWorld() throws Exception {
         BenchmarkState state = mPerfStatusReporter.getBenchmarkState();