feat: Replace encryption methods/key generation from EncryptionUtil with AtChops

[murali-shris]

- What I did

• Replace encryption related methods used from EncryptionUtils with AtChops method
• removed unnecessary exception/code blocks
  - How I did it
• Removed AtKeyException block in shared_key_decryption.dart since AtChops decrypt method doesn't throw this exception
• Removed getting currentAtSignEncryptionPublicKey in abstract_atkey_encryption.dart since at_chops instance will have the encryption public key set at instantiation.
• Replace EncryptionUtil.encryptKey with AtChops.encryptString in abstract_atkey_encryption.dart, self_key_encryption.dart, shared_key_encryption.dart
• Added unit test at_chops_encryption_util_compatibility_test.dart to verify compatibilty check between values encrypted with EncryptionUtil and decrypted using AtChops and vice versa.
• Removed couple of tests from encryption_service_test.dart since they are covered in new tests
  - How to verify it
• unit and functional tests should pass
• @purnimavenkatasubbu Verified data sharing in atmospherePro and atBuzz apps.

[murali-shris]

This pull request addresses two sub tasks in #1145

1. self_key_encryption.dart,shared_key_encryption.dart
   EncryptionUtil.encryptValue
   replace with at_chops.encryptString
2. abstract_atkey_encryption.dart --> createMyCopyOfSharedSymmetricKey
   EncryptionUtil.generateAESKey() - replace with AtChopUtil