[Voice Broadcast] We should not be able to start broadcasting if there is already a live broadcast in the room

### Steps to reproduce

1. Start a VB from a device A in a room
2. Start a VB from a device B on Android in the same room

### Outcome

#### What did you expect?

A dialog should be shown telling that it is not possible to start a new VB

#### What happened instead?

A new VB has been started, so there are two VB at the same time

### Your phone model

_No response_

### Operating system version

_No response_

### Application version and app store

develop

### Homeserver

_No response_

### Will you send logs?

No

### Are you willing to provide a PR?

No