[Android] Fix old root page dispose when main page change (#4707) Fixes #4706 Fixes...

[Android] Fix old root page dispose when main page change (#4707) Fixes #4706 Fixes #4325

* Android - Fix old root page dispose when main page change.

* Revert dead code cleanup.

* Use direct cast for IApplicationController

Co-Authored-By: kvpt <kevin.petit@outlook.com>
* Use direct cast for IApplicationController (bis).

* Unsubscribe the event handler on the right application object.

* Prevent disruptive layout request from toolbar + Revert renderer clearing modification.

* If the page change, do nothing (dont dispose).

* Unsubscribe activity from application on destroy.

* Paused activity must not react to application changes

* Use nameof.