Remove some uses of top-level panels in our examples (#7729)

We're phasing out top-level panels (panels that use `Context` directly,
instead of being inside another `Ui`).
As a first step, stop using them in our demo library and application.

* Part of https://github.com/emilk/egui/issues/3524

examples/hello_android/src/lib.rs

@@ -45,6 +45,8 @@ impl eframe::App for MyApp {
             ui.set_height(32.0);
         });
 
-        self.demo.ui(ctx);
+        egui::CentralPanel::default().show(ctx, |ui| {
+            self.demo.ui(ui);
+        });
     }
 }