Fix run_on_demand exiting on consequent call

Fixes #3284.

src/platform_impl/android/mod.rs

@@ -713,6 +713,10 @@ impl<T: 'static> EventLoopWindowTarget<T> {
         self.exit.set(true)
     }
 
+    pub(crate) fn clear_exit(&self) {
+        self.exit.set(false)
+    }
+
     pub(crate) fn exiting(&self) -> bool {
         self.exit.get()
     }