styles: Update Switch colors and spinning-progress image.

This changes both the thumbColor and trackColor of the ZulipSwitch
component, as the previous thumbColor was the default one, but it
matched with the old colors eliminating the need of passing a thumb-
Color prop.

Also, the spinning-image is updated to use blue color (the same color
used in the logo but with different alpha values) instead of green.

src/common/ZulipSwitch.js

@@ -1,6 +1,7 @@
 /* @flow strict-local */
 import React, { PureComponent } from 'react';
 import { Switch } from 'react-native';
+import Color from 'color';
 import { BRAND_COLOR } from '../styles';
 
 type Props = $ReadOnly<{|
@@ -24,14 +25,16 @@ export default class ZulipSwitch extends PureComponent<Props> {
 
   render() {
     const { disabled, onValueChange, value } = this.props;
-
     return (
       <Switch
         value={value}
         trackColor={{
           false: 'hsl(0, 0%, 86%)',
-          true: BRAND_COLOR,
+          true: Color(BRAND_COLOR)
+            .fade(0.7)
+            .toString(),
         }}
+        thumbColor={value ? BRAND_COLOR : '#f1f1f1'}
         onValueChange={onValueChange}
         disabled={disabled}
       />