[google_maps_flutter] Replace deprecated APIs

[stuartmorgan-g]

This fixes two sets of API deprecations:

• Within the package, cloudMapId was deprecated in the platform interface, and replaced with mapId. This updates everything that's not client-facing to use the mapId name instead (except for local variables in native code, where it ensure there's no potential confusion with the plugin-level map ID that tracks map instances in the plugin).
• Color.value is deprecated. Rather than just replace it with toARGB(), this adds structured data to the Pigeon interface, so that it's not relying on magic knowledge on both sides of the interface that the ints are ARGB:
  • On Android, where the native SDK color representation is just a 32-bit ARGB, this adds a wrapper class to make that explicit in the Pigeon API surface.
  • On iOS, where UIColor uses four doubles, this passes the underlying Color doubles directly via the wrapper class, so that it's not lossy.
  • For legacy JSON representation still used for heatmaps, it just continues to use the equivalent toARGB(). That will change once heatmaps are converted to Pigeon.

This also annotates a couple of intentional internal uses of the deprecated legacy renderer type so that it won't show up in future repository deprecation audits.

Also pays down some tech debt by renaming the now-very-poorly-named FLTGoogleMapJSONConversions.* file to FGMConversionUtils, since it is now mostly Pigeon type conversions. It also renames the legacy FLTGoogleMapJSONConversions bag-of-class-methods class to reflect that it is now only used for heatmaps, the last type that still uses JSON serialization in the Pigeon interface
(flutter/flutter#117907).

Part of flutter/flutter#159739

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
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Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³

[gemini-code-assist]

Code Review

This pull request successfully replaces deprecated APIs, namely cloudMapId and Color.value, across the google_maps_flutter package and its platform implementations. The introduction of a structured PlatformColor type in Pigeon is a significant improvement for type safety and clarity between Dart and native code. The file and class renames to address technical debt are also valuable changes. The overall implementation is systematic and well-executed. I have a few minor suggestions to enhance code clarity by renaming some local variables to align with the new mapId property, which would further improve consistency.