Store videoplayback brakes on volume change

**Problem description**
If i change the volume of a video the player pauses and is unable to play again until i delete the folder:
`/home/user/.var/app/com.valvesoftware.Steam/.local/share/Steam/config/htmlcache`

Deleting all files in `/home/user/.var/app/com.valvesoftware.Steam/.local/share/Steam` is the same like just deleting `htmlcache`.

**Distribution name and version where applicable**
Solus 4.8

**Flatpak info**
Flatpak 1.16.0

flatpak --gl-drivers
- default
- host

**Does this issue reproduce with native Steam**
No. Native Steam works fine.

Reproduce the volume-change failure on Solus 4.8 with Flatpak 1.16.0 and compare it with native Steam, using the reported htmlcache path under /home/user/.var/app/com.valvesoftware.Steam/.local/share/Steam/config/htmlcache. Done means changing volume no longer leaves video playback paused or unable to resume without deleting the cache.

Written by the indexing model from the issue text.