Remove the duplicate code of decrypting the encrypted "defaultEncryptionPrivateKey" and "selfEncryptionKey" in apkam flow

[sitaram-kalluri]

Describe the bug

In the APKAM flow, when an enrollment is approved, the app requesting enrollment access retrieves the "defaultEncryptionPrivateKey" and "defaultSelfEncryptionKey" using the "keys" verb.

Currently, the logic for decrypting the encrypted keys is duplicated in:

• at_libraries/packages/at_onboarding_cli/lib/src/onboard/at_onboarding_service_impl.dart (_getEncryptionPrivateKeyFromServer and _getSelfEncryptionKeyFromServer)
• at_client mobile->at_auth_service_impl.dart (_getDefaultEncryptionPrivateKey and _getDefaultSelfEncryptionKey)
• at_end2end_test/test/enrollment_setup.dart (getDefaultEncryptionPrivateKey and getDefaultSelfEncryptionKey)

To simplify code maintenance, consolidate this logic into a common location, allowing the methods to be reused.