trackScreenChanges: leave BugSnag breadcrumb on navigation

This commit causes trackScreenChanges to send a breadcrumb of type
`navigation` (the only reserved key in this object) and then send the
previous screen as well.

[GitHub] This commit resolves #1800.

Author: rye

source/app.js

@@ -11,6 +11,7 @@ import OneSignal from 'react-native-onesignal'
 import React from 'react'
 import {Provider} from 'react-redux'
 import {store} from './flux'
+import bugsnag from './bugsnag'
 import {tracker} from './analytics'
 import {AppNavigator} from './navigation'
 import type {NavigationState} from 'react-navigation'
@@ -72,6 +73,11 @@ export default class App extends React.Component {
     if (currentScreen !== prevScreen) {
       tracker.trackScreenView(currentScreen)
     }
+
+    bugsnag.leaveBreadcrumb(`Navigated from to ${currentScreen}.`, {
+      type: 'navigation',
+      previousScreen: prevousScreen,
+    })
   }
 
   render() {