Add Appearance module

Summary:
Android implementation of the Appearance native module. Exposes the user's preferred color scheme: "dark" for Night theme ON, "light" for Night theme OFF.

Emits a `appearanceChanged` event when the current uiMode configuration changes.

To make your app handle Night mode changes, make sure to do the following:

* Declare your Activity can handle uiMode configuration changes (https://developer.android.com/preview/features/darktheme#java):
```
android:configChanges="uiMode"
```
* Make sure to pass the configuration changed activity lifecycle callback from your ReactActivity:
```
Override
protected void onConfigurationChanged(Configuration newConfig) {
    super.onConfigurationChanged();

    if (mReactInstanceManager != null) {
        mReactInstanceManager.onConfigurationChanged(newConfig);
    }
}
```

### RNTester

Adds the AppearanceExample to RNTester on Android.

Changelog:

[Android][Added] - New Appearance module exposes the user's current Night theme preference

Reviewed By: makovkastar

Differential Revision: D16942161

fbshipit-source-id: d24a8ff800a1c5f70f4efdec6891396c2078067e

ReactAndroid/src/main/java/com/facebook/react/shell/MainReactPackage.java

@@ -16,6 +16,7 @@ import com.facebook.react.module.annotations.ReactModuleList;
 import com.facebook.react.module.model.ReactModuleInfo;
 import com.facebook.react.module.model.ReactModuleInfoProvider;
 import com.facebook.react.modules.accessibilityinfo.AccessibilityInfoModule;
+import com.facebook.react.modules.appearance.AppearanceModule;
 import com.facebook.react.modules.appstate.AppStateModule;
 import com.facebook.react.modules.blob.BlobModule;
 import com.facebook.react.modules.blob.FileReaderModule;
@@ -71,6 +72,7 @@ import java.util.Map;
 @ReactModuleList(
     nativeModules = {
       AccessibilityInfoModule.class,
+      AppearanceModule.class,
       AppStateModule.class,
       BlobModule.class,
       FileReaderModule.class,
@@ -112,6 +114,8 @@ public class MainReactPackage extends TurboReactPackage {
     switch (name) {
       case AccessibilityInfoModule.NAME:
         return new AccessibilityInfoModule(context);
+      case AppearanceModule.NAME:
+        return new AppearanceModule(context);
       case AppStateModule.NAME:
         return new AppStateModule(context);
       case BlobModule.NAME:
@@ -210,6 +214,7 @@ public class MainReactPackage extends TurboReactPackage {
       Class<? extends NativeModule>[] moduleList =
           new Class[] {
             AccessibilityInfoModule.class,
+            AppearanceModule.class,
             AppStateModule.class,
             BlobModule.class,
             FileReaderModule.class,