[Android]: Missing support for orientation specific splash screen

[Android]: When setting a splashscreen via the theme, there is still a visible blank screen after that before the app window shows. This is because the Qt window is created later and there is a delay before something is shown on screen.

We have the splash screen mechanism in AndroidManifest.xml to work around this, but this does not support having different splash screens for portrait and landscape, which makes it unusable for applications that support auto-orientation.

We need to implement support for this so that those applications can also use the functioning splash screen mechanism.