Since Froyo, Android has had a Dark Mode feature. We should implement it.

This Java code detects if it's enabled:

(mContext.getResources().getConfiguration().uiMode & Configuration.UI_MODE_NIGHT_MASK) == Configuration.UI_MODE_NIGHT_YES;

The activity or fragment's onConfigurationChange apparently triggers when this changes.

We should propagate this state to the MediaQuery somehow (via a system channel, I guess, though we would need to find a way to send the data early in the process so that it's available on first build).

MaterialApp should switch between its regular theme and a darkTheme automatically based on this setting.

cc @HansMuller, @matthew-carroll, @jason-simmons