[perf] ensure mock auto-closeable is closed (#8133)

`MockitoAnnotations.openMocks` returns an auto-closeable that a
`try`-with-resources statement ensures will get closed.

Follow-up from #8111 and
related to #8110.

(Also some opportunistic unused import cleanup.)

---

- [x] I’ve reviewed the contributor guide and applied the relevant
portions to this PR.

<details>
  <summary>Contribution guidelines:</summary><br>

- See our [contributor
guide]([https://github.com/dart-lang/sdk/blob/main/CONTRIBUTING.md](https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview)
for general expectations for PRs.
- Larger or significant changes should be discussed in an issue before
creating a PR.
- Dart contributions to our repos should follow the [Dart style
guide](https://dart.dev/guides/language/effective-dart) and use `dart
format`.
- Java and Kotlin contributions should strive to follow Java and Kotlin
best practices
([discussion](#8098)).
</details>

Author: pq

flutter-idea/src/io/flutter/survey/FlutterSurveyNotifications.java

@@ -24,7 +24,6 @@
 import org.jetbrains.annotations.NotNull;
 
 import java.util.concurrent.Executors;
-import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
 
 public class FlutterSurveyNotifications {

flutter-idea/src/io/flutter/view/FlutterView.java

@@ -15,17 +15,14 @@
 import com.intellij.openapi.progress.EmptyProgressIndicator;
 import com.intellij.openapi.progress.ProgressManager;
 import com.intellij.openapi.project.Project;
-import com.intellij.openapi.ui.VerticalFlowLayout;
 import com.intellij.openapi.util.Disposer;
 import com.intellij.openapi.wm.ToolWindow;
 import com.intellij.openapi.wm.ToolWindowManager;
 import com.intellij.openapi.wm.ex.ToolWindowManagerEx;
 import com.intellij.serviceContainer.NonInjectable;
 import com.intellij.ui.components.JBLabel;
-import com.intellij.ui.components.labels.LinkLabel;
 import com.intellij.ui.content.Content;
 import com.intellij.ui.content.ContentManager;
-import com.intellij.util.ui.JBUI;
 import com.intellij.util.ui.UIUtil;
 import io.flutter.FlutterBundle;
 import io.flutter.FlutterUtils;

flutter-idea/testSrc/unit/io/flutter/view/FlutterViewTest.java

@@ -48,27 +48,28 @@ public class FlutterViewTest {
   ViewUtils viewUtilsSpy;
 
   @Before
-  public void setUp() {
-    MockitoAnnotations.openMocks(this);
-
-    // Static mocking for ApplicationManager.
-    mockedStaticApplicationManager = Mockito.mockStatic(ApplicationManager.class);
-    mockedStaticApplicationManager.when(ApplicationManager::getApplication).thenReturn(mockApplication);
-    doAnswer(invocation -> {
-      Runnable runnable = invocation.getArgument(0);
-      if (runnable != null) {
-        runnable.run();
-      }
-      return null;
-    }).when(mockApplication).invokeLater(any(Runnable.class));
-
-    // Mocking for ContentManager.
-    when(mockToolWindow.getContentManager()).thenReturn(mockContentManager);
-    when(mockContentManager.getFactory()).thenReturn(mockContentFactory);
-
-    // Spy on the ViewUtils class.
-    final ViewUtils viewUtils = new ViewUtils();
-     viewUtilsSpy = spy(viewUtils);
+  public void setUp() throws Exception {
+    try (var mocks = MockitoAnnotations.openMocks(this)) {
+
+      // Static mocking for ApplicationManager.
+      mockedStaticApplicationManager = Mockito.mockStatic(ApplicationManager.class);
+      mockedStaticApplicationManager.when(ApplicationManager::getApplication).thenReturn(mockApplication);
+      doAnswer(invocation -> {
+        Runnable runnable = invocation.getArgument(0);
+        if (runnable != null) {
+          runnable.run();
+        }
+        return null;
+      }).when(mockApplication).invokeLater(any(Runnable.class));
+
+      // Mocking for ContentManager.
+      when(mockToolWindow.getContentManager()).thenReturn(mockContentManager);
+      when(mockContentManager.getFactory()).thenReturn(mockContentFactory);
+
+      // Spy on the ViewUtils class.
+      final ViewUtils viewUtils = new ViewUtils();
+      viewUtilsSpy = spy(viewUtils);
+    }
   }
 
   @After