[google_maps_flutter] Fix A2A iOS builds

[stuartmorgan-g]

Unshares GoogleMapsUtilsTrampoline.h and makes the google_maps_flutter_ios version, which only needs to support CocoaPods, unconditionally use the import version that CocoaPods needs. This works around the fact that (as discussed in the issue) we haven't yet found a way to reliably detect which mode this package is being built in that works in all the edge conditions we tested.

google_maps_flutter_ios_sdk* will still be broken for A2A CocoaPods builds but:

• Someone would have to explicitly opt into that, vs just try to use google_maps_flutter.
• There is currently no advantage to using an _sdk* variant in a CocoaPods build.
• Future versions of Flutter will support SwiftPM-based A2A flows.

Given that, fixing this only for google_maps_flutter may be sufficient even in the long term, in addition to addressing the regression in the short term.

Fixes flutter/flutter#183441

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[stuartmorgan-g]

test-exempt: no behavioral change for anything but a specific A2A flow that is out of scope for testing in this repository

[gemini-code-assist]

Code Review

This pull request addresses an issue with iOS add-to-app builds by modifying how GoogleMapsUtils is imported. The GoogleMapsUtilsTrampoline.h header is unshared, and the google_maps_flutter_ios package version is updated to unconditionally use the CocoaPods-style import (@import GoogleMapsUtils;). This is achieved by removing the conditional preprocessor logic from the header and also removing the corresponding FGM_USING_COCOAPODS definition from the podspec. The shared version of GoogleMapsUtilsTrampoline.h is deleted, and the file path is added to unshared_source_files.dart for google_maps_flutter_ios and its _sdk variants to make them responsible for their own versions. The package version for google_maps_flutter_ios is incremented to 2.18.1 and the CHANGELOG.md is updated accordingly.