Preferred light/dark theme?

## Describe the feature

It'd be nice to automatically be able to accommodate a user's preferred light/dark theme preferences.

Perhaps, simplifying the check of `uiMode && UI_MODE_NIGHT_MASK` [on Android](https://developer.android.com/reference/android/content/res/Configuration.html#uiMode) and `traitCollection.userInterfaceStyle` [on iOS](https://developer.apple.com/documentation/uikit/uiuserinterfacestyle).

These both have `undefined`/`unspecified`, `light`, and `dark`/`night`.

Unfortunately, the [built-in react-native uiMode](https://github.com/facebook/react-native/blob/master/ReactAndroid/src/main/java/com/facebook/react/modules/systeminfo/AndroidInfoModule.java#L47) doesn't give this information.

## Motivation

With iOS 13 and Android Q making this sort of theming more integrated (and Google and Apple's own apps supporting the ability to switch at the OS level), it'd be nice to automatically detect what theme the user wants.

I'd rather not force a user to fumble through app settings to configure the theme if they've already got an option to specify it at the OS level.

## Related Issues

I didn't see any at first glance, but I may have missed them.

Start by reviewing React Native's AndroidInfoModule entry point and the Android uiMode and iOS traitCollection.userInterfaceStyle APIs cited in the issue. The work is done when the theme provider can distinguish unspecified, light, and dark preferences on both platforms, with behavior covered by appropriate tests.

索引モデルが issue の本文から書いたものです。