portals: org.freedesktop.impl.portal.Settings implementation

[togetherwithasteria]

We can get the color-scheme key from Configuration#uiMode! We can get Configuration from Resources#getConfiguration from Application#getResources!!

int currentNightMode = configuration.uiMode & Configuration.UI_MODE_NIGHT_MASK;
switch (currentNightMode) {
    case Configuration.UI_MODE_NIGHT_NO:
        // Night mode is not active, we're using the light theme
        break;
    case Configuration.UI_MODE_NIGHT_YES:
        // Night mode is active, we're using dark theme
        break;
}