Summary:
This addition adds a check for whether a hex color is 3 or 6 digits. If it is 3 digits, it will expand the short form into 6 digits, just like in CSS. The additional benefit of having the length check is that hex colors of invalid lengths (like 4 digits) will be considered invalid. In CSS, invalid length hex colors returns white, but here I have logged an error through `RCTLogError`.
Closes https://github.com/facebook/react-native/pull/455
Github Author: Arthur Lee <arthur@arthurlee.me>
Test Plan: Tried a couple of colors in the sample app, like `#FACEB0`, `#F00`, etc.
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