flutter#186705)

…ner navigation notification

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When propagate the notifcation from the inner nested navigator, it
didn't take the PopScope in between the two navigator into account.

if the widget tree looks like this
```
Navigator1
   PopScope(canpop: false)
       Navigator2
```
when the navigator 2 report canHandlePop = false, the Navigator1
receives it but it just check against its NavigatorState.canPop, which
doesn't take the PopScope in between into account.

This pr changes the logic that it now also check the modalroute's
popdisposition

fixes flutter#181945

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