Fix ErrorProne finding in WmTests

Fixes:
out/soong/.intermediates/frameworks/base/services/tests/wmtests/WmTests/android_common/errorprone/srcjars/frameworks/base/services/tests/wmtests/src/com/android/server/wm/LetterboxConfigurationPersisterTest.java:75: error: [JUnit4TearDownNotRun] tearDown() method will not be run; please add JUnit's @After annotation

Bug: 258754271
Bug: 256019562
Test: m RUN_ERROR_PRONE=true javac-check lint-check
Change-Id: Iafaf8d43984816e6d5434205d935cc1d8843a34d

services/tests/wmtests/src/com/android/server/wm/LetterboxConfigurationPersisterTest.java

@@ -35,6 +35,7 @@ import com.android.internal.annotations.VisibleForTesting;
 
 import junit.framework.Assert;
 
+import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
 
@@ -72,6 +73,7 @@ public class LetterboxConfigurationPersisterTest {
         mLetterboxConfigurationPersister.start();
     }
 
+    @After
     public void tearDown() throws InterruptedException {
         deleteConfiguration(mLetterboxConfigurationPersister, mPersisterQueue);
         waitForCompletion(mPersisterQueue);