Centralize multi-media controllers

[daohoangson]

Use case

Since https://github.com/daohoangson/flutter_widget_from_html/issues/37, webview support has implemented a workaround to dispose the platform view when user navigate away from the app. The same thing happens with video player / audio player / etc. basically any multi-media contents.

Proposal

The factory should give up control of these widgets to the app so app developer can decide what to do as needed (e.g. navigating to another screen / tab within the same app).