Add clear method in FlutterEngineCache to clear all cached Flutter engines #78420 (#25280)

shell/platform/android/io/flutter/embedding/engine/FlutterEngineCache.java

@@ -82,4 +82,9 @@ public class FlutterEngineCache {
   public void remove(@NonNull String engineId) {
     put(engineId, null);
   }
+
+  /** Removes all {@link FlutterEngine}'s that are currently in the cache. */
+  public void clear() {
+    cachedEngines.clear();
+  }
 }

shell/platform/android/test/io/flutter/embedding/engine/FlutterEngineCacheTest.java

@@ -55,4 +55,26 @@ public class FlutterEngineCacheTest {
     // --- Verify Results ---
     assertNull(cache.get("my_flutter_engine"));
   }
+
+  @Test
+  public void itRemovesAllFlutterEngines() {
+    // --- Test Setup ---
+    FlutterEngine flutterEngine = mock(FlutterEngine.class);
+    FlutterEngine flutterEngine2 = mock(FlutterEngine.class);
+    FlutterEngineCache cache = new FlutterEngineCache();
+
+    // --- Execute Test ---
+    cache.put("my_flutter_engine", flutterEngine);
+    cache.put("my_flutter_engine_2", flutterEngine2);
+
+    // --- Verify Results ---
+    assertEquals(flutterEngine, cache.get("my_flutter_engine"));
+    assertEquals(flutterEngine2, cache.get("my_flutter_engine_2"));
+
+    cache.clear();
+
+    // --- Verify Results ---
+    assertNull(cache.get("my_flutter_engine"));
+    assertNull(cache.get("my_flutter_engine_2"));
+  }
 }