fix(init): Reduce allocations and bytecode instructions during Sentry.init (#5135)

* fix(init): Perform less allocation/bytecode instructions in Sentry.init

* Switch new thread to executorService in AssetsModulesLoader

* activate options earlier

* changelog

* fix: Prevent SpotlightIntegration from being re-added after user removal

Use AtomicBoolean to ensure activate() only loads SpotlightIntegration
once, so users can remove it in their configuration callback without
it being re-added by the second activate() call from Sentry.init().

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>