Use egui_demo_lib in android example (#5780)

* [x] I have followed the instructions in the PR template

examples/hello_android/src/main.rs

@@ -0,0 +1,9 @@
+use hello_android::MyApp;
+
+fn main() -> eframe::Result {
+    eframe::run_native(
+        "hello_android",
+        Default::default(),
+        Box::new(|cc| Ok(Box::new(MyApp::new(cc)))),
+    )
+}