Make 'current_monitor' return 'Option<MonitorHandle>'

On certain platforms window couldn't be on any monitor
resulting in failures of 'current_monitor' function.

Such issue was happening on Wayland, since the window
isn't on any monitor, unless the user has drawn something into it.

Returning 'Option<MonitorHandle>' will give an ability to
handle such situations gracefully by properly indicating that
there's no current monitor.

Fixes #793.

src/platform_impl/android/mod.rs

@@ -391,10 +391,10 @@ impl Window {
         v
     }
 
-    pub fn current_monitor(&self) -> monitor::MonitorHandle {
-        monitor::MonitorHandle {
+    pub fn current_monitor(&self) -> Option<monitor::MonitorHandle> {
+        Some(monitor::MonitorHandle {
             inner: MonitorHandle,
-        }
+        })
     }
 
     pub fn scale_factor(&self) -> f64 {