volume keys do not control volume if app is not in foreground

Bug Report

Setup Description

MPD is running in a Termux session on the phone
MALP is directly connected to the phone via localhost:6600 (streaming not enabled)

Version of M.A.L.P.

1.2.4

Version of MPD

0.21.11

Host Operating System

N/A

Android Version

Android 9 / Pie

Device Manufacturer and model

OnePlus One

Bug Description

Description

The volume keys in Android 9 (and possible older) default to controlling the "cast" volume, rather than the media volume when MALP is not in the foreground (including on the lockscreen).

Here's an example of the volume control on the lock screen:

I suspect this has something to do with how the app's sound is registered, or possible the notification (since that is visible while the app is in the background?). I would be happy to look into the issue / create a PR if someone can point me in the right direction in the codebase.

Reproducible

Yes

Expected Behaviour

Volume keys control the media volume when MALP is in background.

Actual Behaviour

Volume keys control the "cast" volume when MALP is in background.

Log

N/A

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