Fix MapboxMap extension plugin functions throwing exceptions

[kiryldz]

Summary of changes

When using extension functions for camera and gesture plugins (e.g. MapboxMap.flyTo and others) we previously were relying on weak references. That could lead to runtime crash:

checkNotNull(cameraAnimationsPlugin?.get()) {
  "Camera plugin is not added as the part of MapInitOptions for given MapView."
}

not only when camera plugin was excluded (as was per design) but also let's say when MapView was already destroyed and extension function was accessed.

Fixed this by removing weak references and improving warning messages.

User impact (optional)

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

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