Flutter应用启动时背景色改成透明色

Flutter应用在启动时会存在一个过渡背景色，
修改以下四个文件：
1、android/app/src/main/AndroidManifest.xml文件中，<application添加一行：

android:theme="@style/LaunchTheme"

2、android/app/src/main/res/values/styles.xml文件中，修改<item>：

<item name="android:windowBackground">@null</item>

3、android/app/src/main/res/values-night/styles.xml文件中，修改<item>：

<item name="android:windowBackground">@null</item>

4、android/app/src/debug/AndroidManifest.xml文件中，将内容替换以下：

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.photogallery">
   <application
        android:label="图库1.0"
        android:name=".PhotoApplication"
        android:theme="@style/LaunchTheme"
        android:icon="@mipmap/ic_launcher">
        <activity
            android:name=".MainActivity"
            android:exported="true"
            android:launchMode="singleTop"
            android:hardwareAccelerated="true"
            android:windowSoftInputMode="adjustResize">
            <!-- Specifies an Android theme to apply to this Activity as soon as
                 the Android process has started. This theme is visible to the user
                 while the Flutter UI initializes. After that, this theme continues
                 to determine the Window background behind the Flutter UI. -->
            <meta-data
              android:name="io.flutter.embedding.android.NormalTheme"
              android:resource="@style/NormalTheme"
              />
            <intent-filter>
                <action android:name="android.intent.action.MAIN"/>
                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>
        </activity>
        <!-- Don't delete the meta-data below.
             This is used by the Flutter tool to generate GeneratedPluginRegistrant.java -->
        <meta-data
            android:name="flutterEmbedding"
            android:value="2" />
    </application>
</manifest>

这个文件修改只是为了本地调试时方便看效果。