Fix memory leak

[leonardo-fernandes]

Fix memory leak - initially reported in apache#290

Whenever closing an InAppBrowser instance, a webview was left in memory with about:blank page.
This change fixes the issue by destroying and freeing the inAppWebView object.

Platforms affected

• Android
• iOS

Motivation and Context

We have detected that a web view is always left open in memory after the InAppBrowser is closed.
apache#290

Description

We have changed the code to destroy the web view.

Testing

The changes were tested with MABS 7.2.

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary