[google_maps_flutter] improvements to google maps state handling

[illuminati1911]

• Added checks to Google Map Controller for map mounted state. This is done to prevent PlatformException when calling methods of already unmounted map instance.
• Improved handling (creation/removal) of tile overlays

Related issue:
flutter/flutter#43785

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[illuminati1911]

Removing this as it seems to be unnecessary when the missing widget.tileOverlays is added to the google map. Also this may have cause this issue:
flutter/flutter#43785 (comment)

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[ChristianEdwardPadilla]

FYI this might help with another issue: flutter/flutter#115283

Which has an older attempted fix PR that you might be able to glean something useful from:
#4281

[stuartmorgan]

Could you elaborate on the goal of this PR? The issue the PR links to appears to be about a case where an internal callback was being run against an unmounted map, and the issue linked in the comment above is also about callbacks, meaning in these cases it's not clear that plugin clients are involved. This PR, however, is putting exceptions on client-facing APIs. Isn't the way for clients to not have this problem just to not call the methods after unmounting the widget, which they can already do?

Is the goal just to improve the error messages? If so, it seems like these should be Errors, not Exceptions, as it seems like a programming error to be calling them in the first place.

[stuartmorgan]

What is the issue this change corresponds to? It's not at all clear to me what this is fixing, or why it is necessary specifically for tile overlays and no other types.