Fixes inconsistencies in the class and file naming in google_maps_flutter_ios:

• Standardizes on FGM instead of a mix of FGM and the older FLT.
• Removes GoogleMaps from most class names, as now that we are using FGM we don't need to further specific that it's related to Google Maps, and removing that makes it much easier to see the useful part of the name.
• Ensures that all files have a prefix matching their class, for consistency.
• Removes prefixing from all test classes and files, since tests don't need prefixes as they aren't part of client builds.

Part of flutter/flutter#102601

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