Control volume of Android tablet

I have installed Home Assistant LoveLace on a Synology Nas in a docker container.
I am running the webpage on a Android tablet with Fully Kiosk browser.
With a single button i can start a radio app and it starts playing through the bluetooth speakers.
Is it possible to put a volumecontrol card on the dashboard to control the volume of the android tablet?

I found two ways to do this. One with the HA Companion App, the other without.

With app:

You could define helpers which trigger these notification commands, which are processed by the app on your tablet.

Without app:
This guy ( How to control the volume of an android device from a pc - Android Enthusiasts Stack Exchange ) wrote a simple app ( https://play.google.com/store/apps/details?id=com.tanaka42.webremotevolumecontrol ) which allows you to control the device volume by HTTP requests. To send the requests from HA, you could add this to your configuration.yaml:

rest_command:
  android_vol_up:
    url: "http://REPLACE_WITH_YOUR_IP:9000/volume-up"
    method: post
  android_vol_down:
    url: "http://REPLACE_WITH_YOUR_IP:9000/volume-down"
    method: post

as described here: RESTful Command - Home Assistant

And again, you define helpers & automations to trigger these services.

VOL+ and VOL- would be two buttons on your dashboard. Which is suboptimal: Best interface would be a slider. Which is possible. Lights and covers have sliders after all. But you’d have to bind it somehow. I didn’t do it. Two buttons are enough for me. I suppose getting a working slider shouldn’t be too hard with the first method. But since I am controlling an outdated phone which does not support the HA app (but only supports the volume-over-http app), I am stuck with the second method.