Fix sharing media inside the app (#7016)
When a `MediaSource` is disposed, the local temp file associated to it will be deleted from disk.
When we try sharing a media from the media viewer screen to another room, this will dismiss the screen, which in turn will dispose of the media source and remove the file, which then fails to be shared.
If we copy it first and then make sure we delete it after trying to send it, it works fine.
