Fix: keepAlive not working when URL changed

[Hamed233]

Issue

When using InAppWebViewKeepAlive, if the URL is changed from the initial URL and the WebView is removed and re-added to the widget tree, it loads the initial URL instead of the last URL that was navigated to.

Steps to reproduce:

1. Load https://jumble.social/ and set keepAlive = InAppWebViewKeepAlive()
2. Navigate to a different URL, like https://jumble.social/notes/bbb
3. Close the WebView
4. Reopen the WebView with the same InAppWebViewKeepAlive object
5. The WebView loads https://jumble.social/ instead of https://jumble.social/notes/bbb

Fix

This PR adds URL tracking to the keepAlive functionality:

1. Added a currentUrl property to the InAppWebViewControllerKeepAliveProps class
2. Updated the onLoadStop event handlers in Android and iOS implementations to save the current URL to the keepAlive properties
3. Modified the controller initialization code to restore the last URL when reusing a keepAlive object

Fixes #2613

[begovsky]

@Hamed233 Please check my comment here:
#2613 (comment)

[aceHubert]

Missing a import