[google_maps_flutter_android] Batch clustered marker operations

[elitree]

Note: this PR attempts to use the same strategy from the Google Maps Flutter Web PR #10562 and apply it to Google Maps Flutter Android.

Summary

• Adds batch addItems() and removeItems() methods to ClusterManagersController
• Refactors MarkersController to batch add/remove/change operations for clustered markers
• Reduces redundant re-clustering calls when multiple markers are added/removed/changed at once

This improves performance when working with large numbers of clustered markers by calling ClusterManager.cluster() once per cluster manager instead of once per marker.

Test plan

• Added unit tests for batch add/remove operations
• Added unit test for batch cluster manager changes
• Existing tests pass

Addresses flutter/flutter#170573

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• 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.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request introduces batch operations for clustered markers to improve performance, which is a great enhancement. The changes involve adding addItems() and removeItems() methods to ClusterManagersController and refactoring MarkersController to group marker operations by cluster manager and execute them in batches. The new logic is well-tested with new unit tests. My review includes a few suggestions to use Map.computeIfAbsent in MarkersController for more concise and modern Java code.