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