Explicitly null map reference from renderer

[kiryldz]

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Fixes: < Link to related issues that will be fixed by this pull request, if they exist >

Pull request checklist:

• Briefly describe the changes in this PR.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality. If tests were not written, please explain why.
• Optimize code for java consumption (@JvmOverloads, @file:JvmName, etc).
• Add example if relevant.
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• Add an entry inside this element for inclusion in the mapbox-maps-android changelog: <changelog>Fix native memory leak by explicitly nulling map reference from renderer.</changelog>.

Summary of changes

Fix native memory leak by explicitly nulling map reference from renderer.
Additionally refactor code a bit by making MapController more responsible for storing main strong link to MapInterface.

Memory consumption looks now after the fix

User impact (optional)