rn,settings: set the placeholder text color

react/features/settings/components/native/FormRow.js

 
 import { translate } from '../../../base/i18n';
 
-import styles, { ANDROID_UNDERLINE_COLOR } from './styles';
+import styles, { ANDROID_UNDERLINE_COLOR, PLACEHOLDER_COLOR } from './styles';
 
 /**
  * The type of the React {@code Component} props of {@link FormRow}
             switch (field.type.displayName) {
             case 'TextInput':
                 return {
+                    placeholderTextColor: PLACEHOLDER_COLOR,
                     style: styles.textInputField,
                     underlineColorAndroid: ANDROID_UNDERLINE_COLOR
                 };

react/features/settings/components/native/styles.js

 } from '../../../base/styles';
 
 export const ANDROID_UNDERLINE_COLOR = 'transparent';
+export const PLACEHOLDER_COLOR = ColorPalette.lightGrey;
+
 const TEXT_SIZE = 17;
 
 /**