Make ContentRectChanged event a non_exhaustive struct

Just to keep open the ability to add state to the event in the future
without needing an API break.

android-activity/src/game_activity/mod.rs

@@ -241,7 +241,7 @@ impl AndroidAppInner {
                                         MainEvent::RedrawNeeded {}
                                     }
                                     ffi::NativeAppGlueAppCmd_APP_CMD_CONTENT_RECT_CHANGED => {
-                                        MainEvent::ContentRectChanged
+                                        MainEvent::ContentRectChanged {}
                                     }
                                     ffi::NativeAppGlueAppCmd_APP_CMD_GAINED_FOCUS => {
                                         MainEvent::GainedFocus

android-activity/src/lib.rs

@@ -137,7 +137,8 @@ pub enum MainEvent<'a> {
     /// Command from main thread: the content area of the window has changed,
     /// such as from the soft input window being shown or hidden.  You can
     /// get the new content rect by calling [`AndroidApp::content_rect()`]
-    ContentRectChanged,
+    #[non_exhaustive]
+    ContentRectChanged {},
 
     /// Command from main thread: the app's activity window has gained
     /// input focus.

android-activity/src/native_activity/mod.rs

@@ -245,7 +245,7 @@ impl AndroidAppInner {
                                         Some(MainEvent::RedrawNeeded {})
                                     }
                                     ffi::APP_CMD_CONTENT_RECT_CHANGED => {
-                                        Some(MainEvent::ContentRectChanged)
+                                        Some(MainEvent::ContentRectChanged {})
                                     }
                                     ffi::APP_CMD_GAINED_FOCUS => Some(MainEvent::GainedFocus),
                                     ffi::APP_CMD_LOST_FOCUS => Some(MainEvent::LostFocus),