[RN] Increase contrast and consistency in LoadingIndicator use cases

react/features/base/react/components/native/LoadingIndicator.js

 import React, { Component } from 'react';
 import { ActivityIndicator } from 'react-native';
 
+import { ColorPalette } from '../../../styles';
+
 /**
  * An animated, large react-native {@link ActivityIndicator} which is considered
  * a suitable visualization of long-running processes with indeterminate amounts
         return (
             <ActivityIndicator
                 animating = { true }
-                size = { 'large' } />
+                color = { ColorPalette.white }
+                size = { 'large' }
+                { ...this.props } />
         );
     }
 }

react/features/welcome/components/WelcomePage.native.js

             // modify non-native children.
             children = (
                 <View>
-                    <LoadingIndicator />
+                    <LoadingIndicator color = { styles.buttonText.color } />
                 </View>
             );
         } else {

react/features/welcome/components/styles.js

      */
     buttonText: {
         alignSelf: 'center',
-        color: '#00ccff',
+        color: ColorPalette.blue,
         fontSize: 18
     },