fixes for Android transparency and add removeAsset method

example/android/app/src/main/res/values/styles.xml

@@ -13,6 +13,6 @@
          
          This Theme is only used starting with V2 of Flutter's Android embedding. -->
     <style name="NormalTheme" parent="@android:style/Theme.Light.NoTitleBar">
-        <item name="android:windowBackground">?android:colorBackground</item>
+        <item name="android:windowBackground">@android:color/transparent</item>
     </style>
 </resources>