fix: Retry restore key creation without cloud backup when E2EE is unavailable - #224

WHY

createRestoreKey() swallows E2eeUnavailableException in a generic catch (e: Exception), so on a device that doesn't meet the end-to-end encryption requirements no restore key is created and the failure is silent. The implementation guide asks for the opposite:

To ensure that Restore Credentials are created in all cases, you must handle the E2eeUnavailableException explicitly by calling createCredential with isCloudBackupEnabled set to true. If E2eeUnavailableException is thrown, catch and call createCredential again with isCloudBackupEnabled set to false.

WHAT

This PR adds that retry. isCloudBackupEnabled is a constructor argument of CreateRestoreCredentialRequest rather than a field of requestJson, so the second attempt builds a new request with it set to false; the first attempt keeps the default of true.