build(android): drop unused Xamarin.AndroidX.SplashScreen resources

- PostIt.Android.csproj: drop Xamarin.AndroidX.Core.SplashScreen and
  Xamarin.AndroidX.Browser. Neither is pulled transitively by
  Avalonia.Android 12.1.1 (its nuspec declares only AppCompat +
  Window), and they are not referenced anywhere in PostIt code.
- Resources/values-v31/styles.xml: drop the windowSplashScreen*
  items and the 'postSplashScreenTheme' reference, which require
  the dropped SplashScreen package. We fall back to the system
  splash screen on Android 12+ devices; a custom one can come
  back when we add it on purpose.

src/PostIt/PostIt.Android/PostIt.Android.csproj

@@ -24,8 +24,6 @@
   </ItemGroup>
   <ItemGroup>
     <PackageReference Include="Avalonia.Android" />
-    <PackageReference Include="Xamarin.AndroidX.Core.SplashScreen" />
-    <PackageReference Include="Xamarin.AndroidX.Browser" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\PostIt\PostIt.csproj" />

src/PostIt/PostIt.Android/Resources/values-v31/styles.xml

@@ -8,14 +8,5 @@
     <item name="android:windowActionBar">false</item>
     <item name="android:windowBackground">@null</item>
     <item name="android:windowNoTitle">true</item>
-    <item name="android:windowSplashScreenBackground">@color/splash_background</item>
-    <item name="android:windowSplashScreenAnimatedIcon">@drawable/avalonia_anim</item>
-    <item name="android:windowSplashScreenAnimationDuration">1000</item>
-    <item name="postSplashScreenTheme">@style/MyTheme.Main</item>
-
-  </style>
-  <style name="MyTheme.Main"
-         parent ="MyTheme.NoActionBar">
-    <item name="android:windowIsTranslucent">true</item>
   </style>
 </resources>