Reset SafetyCenterManagerWrapper instance between tests.

This is because some other tests may set a mock on this static value, which will persist across test cases (whereas in this test we want to test the real thing).

Bug: 335557039
Change-Id: I14fc81922761159241636c78dbd47fce7bfeb533
Test: atest SafetyCenterManagerWrapperTest

tests/unit/src/com/android/settings/safetycenter/SafetyCenterManagerWrapperTest.java

@@ -40,6 +40,7 @@ public class SafetyCenterManagerWrapperTest {
     @Before
     public void setUp() {
         MockitoAnnotations.initMocks(this);
+        SafetyCenterManagerWrapper.sInstance = null;
     }
 
     @Test