Android: Differ between fullscreen and maximized windows

Currently, we set the flag ShowIsFullscreen in Android, but what we are really doing is maximizing all windows, because the status bar is still visible unless you remove it in the AndroidManifest.

In Qt terms, the logical interpretation is that:
show(): Whatever is the platform default
showMaximized(): Fills the area of the screen which is not used for system UI
showFullscreen(): Fills the entire screen

We should follow this on Android as well. This means that instead of "ShowIsFullscreen", we should have "ShowIsMaximized" to ensure that all windows are maximized by default.

In addition, showFullscreen() should actually hide the system UI elements that we are able to hide, such as the status bar.

A way to do this which is compatible with our minimum SDK version is:

getWindow().addFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN);