build: migrate to AGP 9 and built-in Kotlin support

[kikoso]

This PR updates the project to correctly use AGP 9.0+ features and conventions:

• Removes the deprecated kotlin-android plugin and migrates to built-in Kotlin support.
• Removes deprecated srcDir usages in favor of directories.add().
• Adds explicitly generated source paths to the kotlin sourceSet so AttributionId builds properly.
• Fixes 'Suspicious receiver type' warnings by moving kotlin and screenshotTests blocks outside the android block to the correct project scope.
• Suppresses UnstableApiUsage warning for experimentalProperties usage.

[googlemaps-bot]

Code Coverage

Overall Project	24.82% -0.66%	❌
Files changed	0%	❌

File	Coverage
GroundOverlayActivity.kt	0% -7.31%	❌
MarkerClusteringActivity.kt	0% -3.7%	❌

[kikoso]

Holding on the BoM update, since there seems to be an issue with the Marker InfoWindow.