[google_maps_flutter] Call platform.dispose on GoogleMap widget dispose.

[ditman]

Description

This change uses the dispose method introduced in #2908, and calls it on dispose of the GoogleMap widget. This allows the plugin to cleanup resources once the framework is done with the widget rendering the map.

(Note: this PR will fail until the one above lands!)

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
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Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• No, this is not a breaking change.

[cyanglaz]

LGTM