_OpenContainerRoute extends PageRoute

[danielgorka]

Change _OpenContainerRoute to extends PageRoute.
When extending ModalRoute the hero animations are not working, because hero checks for PageRoute.

Issue: #flutter/flutter#51337

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[danielgorka]

@googlebot I signed it!

[goderbauer]

@danielgorka looks like this change is causing test failures. Can you look at those? In addition, this change also needs tests. Can you please add some? Thanks.