ndk-glue: Finish native activity after returning from main()

When an application returns its control flow back from `main()` it is
generally assumed to be done polling on the looper and wishes to
terminate.  Android is made aware of this by calling
`ANativeActivity_finish`, otherwise it'll continue sending input events
and eventually ANR as the app didn't consume them anymore.

ndk-glue/src/lib.rs

@@ -198,7 +198,12 @@ pub unsafe fn init(
             signal_looper_ready.notify_one();
         }
 
-        main()
+        main();
+
+        NATIVE_ACTIVITY
+            .take()
+            .expect("Native activity should not have been taken before main() returns!")
+            .finish()
     });
 
     // Don't return from this function (`ANativeActivity_onCreate`) until the thread