Add an example

It's easier to test your changes if you have an example that you can
execute. Maybe it's at least a little useful to potential users, too.

examples/time_zone.rs

@@ -0,0 +1,9 @@
+/// Prints the current time zone, e.g. "Europe/Paris".
+
+use android_system_properties::AndroidSystemProperties;
+
+fn main() {
+    let android_system_properties = AndroidSystemProperties::new();
+    let tz = android_system_properties.get("persist.sys.timezone");
+    println!("Your time zone is: {}", tz.as_deref().unwrap_or("<unknown>"));
+}