android: migrate to compiled API v31+, with splash screen support

android: migrate to compiled API v31+, with splash screen support

use AndroidX for getting compat support with our min SDK level v23

The splash screen support is now handled directly by android, I
actually noticed that as Flutter complained about the old way
being used and pointed to
http://flutter.dev/go/android-splash-migration
which redirects to
https://docs.flutter.dev/development/ui/advanced/splash-screen

Note, I saw some issues with the icon not showing up on Android 12
while the older versions were OK, seems like it could be:
https://issuetracker.google.com/issues/207095799

But need to closer investigate, for now splash screen is not /that/
essential and the basics should be right, so we better move forward
now and bugfix whatever it is.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>