Moved code dealing with accessibility tree from switch_access.js to new file, automation_manager.js

chrome/browser/resources/chromeos/switch_access/manifest.json.jinja2

 {
 {% if key is defined %}
   "key": "{{key}}",
 {% endif %}
   "manifest_version": 2,
   "name": "TODO: Translated Switch access name",
   "version": "{{set_version}}",
   "description": "TODO: Translated Switch access description",
 {% if is_guest_manifest == '1' %}
   "incognito": "split",
 {% endif %}
   "background": {
     "scripts": [
       "prefs.js",
       "auto_scan_manager.js",
+      "automation_manager.js",
       "keyboard_handler.js",
       "tree_walker.js",
       "switch_access.js",
       "background.js"
     ]
   },
   "permissions": [
     "accessibilityPrivate",
     "storage",
     "tabs"
   ],
   "automation": {
     "desktop": true
   }
 }