Fix memory leak by nullifying lifecycleObserver (#98)

## Pull request

Please ensure this PR targets the **`dev` branch** and follows the
project conventions.
CI already runs linting, formatting, and build checks automatically.

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### Before submitting

- [x] This PR targets the `dev` branch (not `main`)
- [x] Commit messages follow the semantic-release format
- [x] No debug logs or sensitive data included

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### Summary

Fixes a memory leak caused by mapView reference kept by
lifecycleObserver

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### Type of change

- [ ] Feature
- [x] Fix
- [ ] Refactor
- [ ] Internal / CI
- [ ] Documentation

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### Scope

- [x] Android
- [ ] iOS
- [ ] JS
- [ ] Example App
- [ ] Docs

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### Additional notes

There's a memory leak caused by lifecycleObserver apparently because if
keeps a reference of mapView. Nullifying lifecycleObserver fixed it for
me.

The memory leak can be observed by putting GoogleMapsView on a stack or
a modal and then closing and navigating to it multiple times. The
easiest way to reproduce it is to render GoogleMapsView on a flatlist a
few hundred times and to scroll back and forth causing an out of memory
crash.

Author: pinpong

android/src/main/java/com/rngooglemapsplus/GoogleMapsViewImpl.kt

@@ -813,6 +813,7 @@ class GoogleMapsViewImpl(
       if (destroyed) return@onUi
       destroyed = true
       lifecycleObserver?.toDestroyedState()
+      lifecycleObserver = null
       markerBuilder.cancelAllJobs()
       clearMarkers()
       clearPolylines()