#386 hotfix/cp-12672-anrs-on-app-start-cleverpushinit-does-database-and

Fix main-thread disk access during SDK initialization

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Summary by cubic

Moves SharedPreferences and database work off the main thread during SDK initialization to avoid ANRs on app start.

• init() prepares and migrates SharedPreferences on a background executor, then resumes initialization on the main thread.
• Retention cleanup runs on a dedicated database executor instead of the calling thread.
• Keeps the synchronous path when initialization starts off the main thread.