Implement Window::theme and WindowEvent::ThemeChanged on iOS and Android.

[xorgy]

Window::theme and WindowEvent::ThemeChanged are incomplete on some platforms, so I'm adding this here to document platform APIs I'm aware of for supporting these.

on iOS:

Window::theme can likely be implemented with UITraitEnvironment which UIView implements. WindowEvent::ThemeChanged can be triggered by a handler with registerForTraitChanges.

• let's implement this on iOS

on Android:

Window::theme can likely be implemented by looking at Configuration from ContextThemeWrapper.getResources(); this is wired through in android-activity with ConfigurationRef::ui_mode_type.
and WindowEvent::ThemeChanged could be triggered by View.onConfigurationChanged() but this requires the manifest's <activity> element to have an android:configChanges attribute containing uiMode.
android-activity currently wires this through with MainEvent::ConfigChanged.

• let's implement this on Android