[Android] 增加引擎释放接口 (alibaba#1291)

* [Android] 增加引擎释放接口

* 增加注释

Author: 0xZOne

android/src/main/java/com/idlefish/flutterboost/FlutterBoost.java

@@ -83,6 +83,21 @@ public void setup(Application application, FlutterBoostDelegate delegate, Callba
         setupActivityLifecycleCallback(application, isBackForegroundEventOverridden);
     }
 
+    /**
+     *  Releases the engine resource.
+     */
+    public void tearDown() {
+        FlutterEngine engine = getEngine();
+        if (engine != null) {
+            engine.destroy();
+            FlutterEngineCache.getInstance().remove(ENGINE_ID);
+        }
+        topActivity = null;
+        plugin = null;
+        isBackForegroundEventOverridden = false;
+        isAppInBackground = false;
+    }
+
     /**
      * Gets the FlutterBoostPlugin.
      *