Splash Screen Background Color for Dark Mode

[ghnp5]

#816 allows setting a themeColor for Dark Mode.

However, I believe the same should exist for backgroundColor, which sets the background color of the Splash Screen.

Currently, even if I'm on Dark Mode, the app shows a light-color screen while the app loads.

So, I propose a backgroundColorDark setting.

FAO @andreban

bubblewrap/packages/core/template_project/app/src/main/AndroidManifest.xml

Lines 135 to 136 in 2395d6a

	<meta-data android:name="android.support.customtabs.trusted.SPLASH_SCREEN_BACKGROUND_COLOR"
	android:resource="@color/backgroundColor"/>

Thank you!