I had the same problem, but finally managed to fix it on my old tablet (Android 7.1.2). The problem appeared to be that HA needs Android Webview. Which is now disabled by default in favor of the Google Web Engine. To disable it go to settings → developer options. If you dont have that go to settings->about and tap tap 5 times on the build number. It will tell you you are now a developer.
From the developer options that now should be available in Settings you can choose “Webview” and there you can select “Android Webview”. If that option isnt available, first install it from play store. After installing you can choose it as said above. After setting the option I rebooted to make sure and both my HA companion and worked after that.
Hope this will help someone!