Android: Allow custom background color

[ox-michaelradionov]

Adds third optional argument to SplashScreen.show() on Android to allow customization of background color. It is important for me because this color appears for a second when keyboard animation happens (it is displayed when space for keyboard is already reserved but opening animation is still happening). By default white background color is used.

Usage:

import android.graphics.Color;

SplashScreen.show(this, getReactInstanceManager(), Color.rgb(255, 0, 0));

[mehcode]

@ox-michaelradionov Would it be possible to get this color from the theme? Or something similar declaratively? I'll merge this if that's non-trivial or doesn't make sense to you.

[ox-michaelradionov]

@mehcode I'm not sure if it is possible, I'll check

[ox-michaelradionov]

@mehcode I've updated the PR to use themes as you suggested.