Enable camera animators to run without storing a reference

[jmkiley]

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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.
• Document any changes to public APIs.
• Apply changelog label ('breaking change', 'bug 🪲', 'build', 'docs', 'feature 🍏', 'performance ⚡', 'testing 💯') or use the label 'skip changelog'
• Add an entry inside this element for inclusion in the mapbox-maps-ios changelog: <changelog>Updated BasicCameraAnimator so that it keeps animators alive while the animator is active.</changelog>.

Summary of changes

This PR stores a reference to an animator within BasicCameraAnimator and sets it to self while the animator is active. If the animation is paused or stopped, that property is set to nil. It uses the approach from @macdrevx's ah/keep-alive branch.

This also updates the CameraAnimatorsExample so that the animators are no longer properties. I also tested out pausing and stopping the animators and different points.

User impact (optional)