Color: the two alpha orderings

Color accepts many input forms, and the alpha channel sits at opposite ends depending on the form. Mixing them up produces "the right color at the wrong opacity" bugs that survive review.

import { Color } from '@nativescript/core';

const c = new Color(100, 255, 100, 100); // 4-arg constructor: (a, r, g, b) — alpha FIRST
c.a;    // 100
c.r;    // 255
c.hex;  // '#FF646464'  — hex string: RRGGBBAA — alpha LAST
c.argb; // 0x64ff6464   — int: 0xAARRGGBB — alpha FIRST