Use ActivityTestRule as a Rule

Rules are rules, and the rules are that @Rule rules go through
ActivityStatement.evaluate, which among other things synchronously
finishes the activity under test even if it was manually started.

Bug: 276843259
Test: atest --iterations 100 ActivityThreadTest
Change-Id: I6fd1bb00b587250192e84bcb70771eda1259d36b

core/tests/coretests/src/android/app/activity/ActivityThreadTest.java

@@ -71,6 +71,7 @@ import androidx.test.runner.AndroidJUnit4;
 import com.android.internal.content.ReferrerIntent;
 
 import org.junit.After;
+import org.junit.Rule;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 
@@ -95,7 +96,8 @@ public class ActivityThreadTest {
     // few sequence numbers the framework used to launch the test activity.
     private static final int BASE_SEQ = 10000;
 
-    private final ActivityTestRule<TestActivity> mActivityTestRule =
+    @Rule
+    public final ActivityTestRule<TestActivity> mActivityTestRule =
             new ActivityTestRule<>(TestActivity.class, true /* initialTouchMode */,
                     false /* launchActivity */);