[google_maps_flutter] Raise MapUsedAfterWidgetDisposedError when map controller used after map disposed

[filiph]

This change introduces a new MapUsedAfterWidgetDisposedError that facilitates debugging when the user calls a method on GoogleMapsController after the associated map has been disposed. This replaces the previous behavior, which would sometimes throw a Platform-side error (MissingPluginException, Unable to establish connection on channel, etc.).

Although technically non-breaking (we're replacing one error with another), this could be disruptive because the new error is raised eagerly and synchronously, as soon as a call is made. Before, the call could have potentially succeeded (?), and the error was asynchronous (and so, potentially, unawaited and ignored?).

Fixes flutter/flutter#43785.

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[filiph]

I was just made aware of #9227, which seems to fix the other aspect of flutter/flutter#43785. Happy to wait with this one until after that one lands.