feat: Android start mode.

demo/Semi.Avalonia.Demo.Android/Resources/drawable/splash_screen.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
+
+  <item>
+    <color android:color="@color/splash_background"/>
+  </item>
+
+  <item android:drawable="@drawable/Icon"
+        android:width="120dp"
+        android:height="120dp"
+        android:gravity="center" />
+
+</layer-list>

demo/Semi.Avalonia.Demo.Android/Resources/values/colors.xml

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+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+  <color name="splash_background">#FFFFFF</color>
+</resources>

demo/Semi.Avalonia.Demo.Android/Resources/values/styles.xml

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+<?xml version="1.0" encoding="utf-8" ?>
+<resources>
+
+  <style name="MyTheme">
+  </style>
+
+  <style name="MyTheme.NoActionBar">
+    <item name="android:windowActionBar">false</item>
+    <item name="android:windowNoTitle">true</item>
+  </style>
+
+  <style name="MyTheme.Splash" parent ="MyTheme.NoActionBar">
+    <item name="android:windowBackground">@drawable/splash_screen</item>
+    <item name="android:windowContentOverlay">@null</item>
+  </style>
+
+</resources>