[google_maps_flutter_android] Avoid redundant ground overlay image updates

[ziyad-aljohani]

interpretGroundOverlayOptions currently converts and applies the image on every ground overlay change, even when only properties such as transparency or position changed. For byte-backed images, this unnecessarily decodes the image and recreates its BitmapDescriptor on the platform thread.

This change stores the last PlatformBitmap in GroundOverlayController and uses its generated deep equality to determine whether the image changed. Property-only updates now skip image conversion and setImage, while genuine image changes continue through the existing path.

Partially addresses flutter/flutter#189702 by removing redundant image work from property-only updates. This optimization does not make multi-overlay swaps transactional.

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Code Review

This pull request modifies the Android implementation of the Google Maps Flutter plugin to avoid recreating ground overlay images when only other overlay properties change. It updates Convert.java, GroundOverlayController.java, and GroundOverlaysController.java to track the current PlatformBitmap and only update the image if it has changed. Additionally, it includes corresponding unit tests in GroundOverlaysControllerTest.java and bumps the package version to 2.19.13. There are no review comments, and I have no feedback to provide.