Replace privileged field with a not_sandboxed() method

`privileged` now only indicates if a client is "sandboxed", i.e. it has
a security context, where the sandbox engine isn't cosmic-panel.

So replace the field with a method that's a bit more descriptive.

src/wayland/handlers/security_context.rs

@@ -44,8 +44,6 @@ impl SecurityContextHandler for State {
                     client_stream,
                     Arc::new(ClientState {
                         security_context: Some(security_context.clone()),
-                        privileged: security_context.sandbox_engine.as_deref()
-                            == Some("com.system76.CosmicPanel"),
                         advertised_drm_node: drm_node,
                         ..new_state
                     }),

src/wayland/handlers/xdg_activation.rs

@@ -39,7 +39,7 @@ impl XdgActivationHandler for State {
             })
             .and_then(|data| {
                 data.downcast_ref::<ClientState>()
-                    .map(|data| data.privileged)
+                    .map(|data| data.not_sandboxed())
             })
             .unwrap_or(false)
         {