[google_maps_flutter] Adopt FFI/JNI Pigeon backend #192720

Now that #119108 is complete (Android was already switched to Kotlin Pigeon generation), and Pigeon 29 has been released with the FFI/JNI backend, we should be able to use google_maps_flutter as one of the initial tests for the new backend (with the eventual goal of switching all plugins over). This is a particularly interesting test case as it has:

• both sync and async methods
• significant use of both HostApi and FlutterApi
• specific use cases where synchronous communication would unlock highly requested functionality, most notably [google_maps_flutter] Add support for custom cluster icon #153092

For each of the two platforms, we should probably do this in two steps:

1. switch for the callback-based API to the new async/await API on the current backend (since FFI/JNI uses that form, so doing it first will reduce changes later), then
2. switch to the FFI/JNI backend.