Use React Image source propType for checkedImage and uncheckedImage

[ysdanielkim]

Since the checkedImage and uncheckedImage is directly put into Image's source attribute, it'd be ideal to have it's type match Image source propType.

ImageSourcePropType is defined here: https://github.com/facebook/react-native/blob/efcdef711eba82b2905d237ee9a3d094652c37ac/Libraries/Image/ImageSourcePropType.js

I can create a pull request if you wish.