feat: Add CallbackActivity for flutter_web_auth_2 and update launcher icons for android

android/app/src/main/AndroidManifest.xml

···
                 <category android:name="android.intent.category.LAUNCHER"/>
             </intent-filter>
         </activity>
+        <activity
+            android:name="com.linusu.flutter_web_auth_2.CallbackActivity"
+            android:exported="true">
+            <intent-filter android:label="flutter_web_auth_2">
+            <action android:name="android.intent.action.VIEW" />
+            <category android:name="android.intent.category.DEFAULT" />
+            <category android:name="android.intent.category.BROWSABLE" />
+            <data android:scheme="grainflutter" />
+            </intent-filter>
+        </activity>
         <!-- Don't delete the meta-data below.
              This is used by the Flutter tool to generate GeneratedPluginRegistrant.java -->
         <meta-data