Refactor: Minor performance improvement of BackHandler.removeEventListener (#34281)

Summary:
I've noticed that `BackHandler.removeEventListener()` performs two same `indexOf()` calls on an array that is not changing. By removing extra `indexOf` we can slightly improve time complexity of `BackHandler.removeEventListener()` from O(2n) to O(n)

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[Android] [Fixed] - Remove extra indexOf call in BackHandler.removeEventListener

Pull Request resolved: https://github.com/facebook/react-native/pull/34281

Test Plan:
1. Add the following code to any function component
```javascript
  BackHandler.addEventListener('hardwareBackPress', () => true).remove();
```

2. Press on hardware back button

Expected result: Application closes

Reviewed By: dmitryrykun

Differential Revision: D38198510

Pulled By: javache

fbshipit-source-id: eab6a57689a536623138a4b3ebddbf9ba87d281f

Libraries/Utilities/BackHandler.android.js

@@ -97,11 +97,9 @@ const BackHandler: TBackHandler = {
     eventName: BackPressEventName,
     handler: () => ?boolean,
   ): void {
-    if (_backPressSubscriptions.indexOf(handler) !== -1) {
-      _backPressSubscriptions.splice(
-        _backPressSubscriptions.indexOf(handler),
-        1,
-      );
+    const index = _backPressSubscriptions.indexOf(handler);
+    if (index !== -1) {
+      _backPressSubscriptions.splice(index, 1);
     }
   },
 };