Custom background image for a room

### Your use case

#### What would you like to do?
I'd like to be able to set a custom image for a given room

#### Why would you like to do it?
To give a better visual feedback on where I'm currently chatting

#### How would you like to achieve it?
It should be a local setting, not stored on server. A background can then be different on each device/client I'm connected

### Have you considered any alternatives?

_No response_

### Additional context

This seems linked to https://github.com/element-hq/element-meta/discussions/625 and https://github.com/element-hq/element-web/issues/5844 but they only talk about Element Web/Desktop.

I know that at least WhatsApp has this feature, and my homeserver's users are missing this feature.

### Are you willing to provide a PR?

No

The issue names no files, tests, or entry points. Start by tracing room presentation and local per-device settings in the Android client; done means a room can display its own selected background image without storing it on the homeserver.

Written by the indexing model from the issue text.