cx.background_executor().spawn(...) -> cx.background_spawn(...) (#25103)

Done automatically with

> ast-grep -p '$A.background_executor().spawn($B)' -r
'$A.background_spawn($B)' --update-all --globs "\!crates/gpui"

Followed by:

* `cargo fmt`
* Unexpected need to remove some trailing whitespace.
* Manually adding imports of `gpui::{AppContext as _}` which provides
`background_spawn`
* Added `AppContext as _` to existing use of `AppContext`

Release Notes:

- N/A

crates/livekit_client/src/remote_video_track_view.rs

@@ -1,7 +1,9 @@
 use crate::track::RemoteVideoTrack;
 use anyhow::Result;
 use futures::StreamExt as _;
-use gpui::{AppContext, Context, Empty, Entity, EventEmitter, IntoElement, Render, Task, Window};
+use gpui::{
+    AppContext as _, Context, Empty, Entity, EventEmitter, IntoElement, Render, Task, Window,
+};
 
 pub struct RemoteVideoTrackView {
     track: RemoteVideoTrack,