Fix isMobileApp() to also apply to Android app (#4892)

* Fix isMobileApp() to also apply to Android app

* Add checking agent to LHS download app link

webapp/components/sidebar_header_dropdown.jsx

@@ -3,6 +3,7 @@
 
 import $ from 'jquery';
 import ReactDOM from 'react-dom';
+import * as UserAgent from 'utils/user_agent.jsx';
 import * as Utils from 'utils/utils.jsx';
 import * as GlobalActions from 'actions/global_actions.jsx';
 
@@ -412,7 +413,7 @@ export default class SidebarHeaderDropdown extends React.Component {
 
         let nativeAppDivider = null;
         let nativeAppLink = null;
-        if (global.window.mm_config.AppDownloadLink) {
+        if (global.window.mm_config.AppDownloadLink && !UserAgent.isMobileApp()) {
             nativeAppDivider = <li className='divider'/>;
             nativeAppLink = (
                 <li>

webapp/utils/user_agent.jsx

@@ -82,7 +82,7 @@ export function isAndroidWeb() {
 // Returns true if and only if the user is using a Mattermost mobile app. This will return false if the user is using the
 // web browser on a mobile device.
 export function isMobileApp() {
-    return userAgent.indexOf('iPhone') !== -1 && userAgent.indexOf('Safari') === -1 && userAgent.indexOf('CriOS') === -1;
+    return isMobile() && !isIosWeb() && !isAndroidWeb();
 }
 
 // Returns true if and only if the user is using Mattermost from either the mobile app or the web browser on a mobile device.