[android] fix MauiSplashScreen on Android 12+ (#8394)

Fixes: #7404
Context: https://developer.android.com/guide/topics/ui/splash-screen#set-theme

Reviewing a lot of the feedback around splash screens, I tried:

1. Rename `splash.svg` to `splash2.svg`
2. Change the .NET splash logo from white to blank.

I found these changes worked fine on Android < 12, but I did not see
my changes on Android 12+!

This was more troubling when I changed the `MauiSplashScreen` `Color`
to white.

After some review, it looks like we need to define:

    <item name="android:windowSplashScreenBackground">@color/maui_splash_color</item>
    <item name="android:windowSplashScreenAnimatedIcon">@drawable/maui_splash</item>

With these changes in place, the splash screen is working for me in
Android 12+.

I believe we implemented `MauiSplashScreen` while Android 12 was still
in beta, so we just didn't notice this behavior. We also were sharing
the same `.svg` file for the app icon and splash screen previously,
which may have confused or hidden this issue.

src/Core/src/Platform/Android/Resources/values/styles.xml

@@ -29,6 +29,12 @@
  <style name="Maui.SplashTheme" parent="Maui.MainTheme.NoActionBar">
  <item name="maui_splash">true</item>
  <item name="android:windowBackground">@drawable/maui_splash</item>
+ <!--
+ Android 12+ specific settings
+ See: https://developer.android.com/guide/topics/ui/splash-screen#set-theme
+ -->
+ <item name="android:windowSplashScreenBackground">@color/maui_splash_color</item>
+ <item name="android:windowSplashScreenAnimatedIcon">@drawable/maui_splash</item>
  </style>
 
  <!-- Themes for Xamarin.Forms backwards compatibility -->