fix: use old keyboardView reference for removing observer

[kirillzyusko]

📜 Description

Use memoized keyboardView reference for removing observer.

💡 Motivation and Context

Since keyboardView is a getter there are some chances that it may return a new reference to a view when we access it.

In some cases it may be a problem, because it can return a reference to a view which doesn't contain an observer yet. As a result when we'll try to remove an observer from a view which doesn't have it -> we'll get a crash.

To prevent a crash in removeKVObserver we start to use _keyboardView instead of keyboardView. It'll give us an access to old view and we can safely remove the observer.

Fixes #155

Potentially also fixes: #152

📢 Changelog

iOS

• use _keyboardView instead of keyboardView when remove observer;

🤔 How Has This Been Tested?

Tested on iPhone 14 (simulator, iOS 16.2) using following code:

<Button title="show image picker" onPress={async () => {
  // You can also use as a promise without 'callback':
  const result = await launchImageLibrary();
}} />

📸 Screenshots (if appropriate):

Before	After
Simulator.Screen.Recording.-.iPhone.14.Pro.-.2023-05-30.at.16.55.42.mp4	Simulator.Screen.Recording.-.iPhone.14.Pro.-.2023-05-30.at.16.54.08.mp4

📝 Checklist

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