TextInput should have placeholder color inherited from style props

[jpbulman]

Currently, to change the color of placeholder text in a TextInput, you need to pass in a color to the placeholderTextColor prop directly.

<TextInput
    style={styles.exampleInput}
    placeholder="This is example text"
    placeholderTextColor="red"
/>
...
const styles = StyleSheet.create({
    exampleInput: {
    }
})

Instead, the TextInput placeholder text color should inherit it from the color in the style by default. For backwards compatibility, it could be that the placeholderTextColor prop still take precedence, but it should default to the color style, like so:

<TextInput
    style={styles.exampleInput}
    placeholder="This is example text"
/>
...
const styles = StyleSheet.create({
    exampleInput: {
        color: "red"
    }
})