perf: improve MarkerComposable rendering performance

[brucemok8]

Summary

This PR improves the rendering performance of MarkerComposable, particularly when displaying a larger number of markers with frequently updating content.

Changes

• Reuse ComposeView instances instead of creating and disposing them for each render. The view is retained and used to generate updated bitmaps when marker keys change, and disposed only when the marker is removed.
• Move renderComposableToBitmapDescriptor() off the main thread to avoid blocking the UI.
• Update rememberComposeBitmapDescriptor to return a nullable type. Callers skip rendering until the bitmap is ready, preventing main thread blocking and allowing markers to appear asynchronously.

Impact

These changes significantly reduce UI jank during marker updates. In testing, scenarios with ~25 dynamically updating markers show noticeably smoother performance.

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