group-pm: Improve the text color on group icons

Determine the color of text on group avatar by using
foregroundColorFromBackground(). This will help users to
read the text properly when the background color is light.

src/common/GroupAvatar.js

@@ -4,7 +4,7 @@ import { Text, StyleSheet, View } from 'react-native';
 
 import type { Node as React$Node } from 'react';
 import Touchable from './Touchable';
-import { colorHashFromString } from '../utils/color';
+import { colorHashFromString, foregroundColorFromBackground } from '../utils/color';
 import { initialsFromString } from '../utils/misc';
 
 const styles = StyleSheet.create({
@@ -52,6 +52,7 @@ export default class GroupAvatar extends PureComponent<Props> {
     };
     const textSize = {
       fontSize: size / 3,
+      color: foregroundColorFromBackground(frameSize.backgroundColor),
     };
 
     return (