fix: Export backupkey_contants and getEncryptedKeysMethod

[sitaram-kalluri]

- What I did

• Uptaking the at_client_mobile 3.2.16 into at_backupkey_flutter results in the following lint warnings:

warning • The ability to import 'KeychainUtil' indirectly is deprecated • lib/services/backupkey_service.dart:13:18 • deprecated_export_use
warning • The ability to import 'KeychainUtil' indirectly is deprecated • lib/services/backupkey_service.dart:19:22 • deprecated_export_use
warning • The ability to import 'BackupKeyConstants' indirectly is deprecated • test/backupkey_service_test.dart:130:24 • deprecated_export_use
warning • The ability to import 'BackupKeyConstants' indirectly is deprecated • test/backupkey_service_test.dart:137:24 • deprecated_export_use
warning • The ability to import 'BackupKeyConstants' indirectly is deprecated • test/backupkey_service_test.dart:144:24 • deprecated_export_use
warning • The ability to import 'BackupKeyConstants' indirectly is deprecated • test/backupkey_service_test.dart:151:24 • deprecated_export_use
warning • The ability to import 'BackupKeyConstants' indirectly is deprecated • test/backupkey_service_test.dart:156:24 • deprecated_export_use

The warnings are because "BackupKeyConstants" and "KeychainUtil" classes are present in the deprecated "at_client_service.dart" file, which has now been replaced by "AtAuthServiceImpl".

To resolve the lint warnings:

• The BackupKeyConstants class has been relocated to the "auth_constants.dart" file, and only this class is exported.
• Most methods in the KeychainUtil class are wrappers around the methods in the already-exported KeychainManager class except for the "getEncryptedKeys" method. So, moved the method into the KeychainManager class.

- How to verify it

• Used dependency_overrides in at_backupkey_flutter and ran flutter analyze and no lint warning are found.

~/IdeaProjects/atsign/widgets/at_widgets/packages/at_backupkey_flutter git:[fix_lint_issues_in_backupkey_flutter]
flutter analyze
Analyzing at_backupkey_flutter...                                       
No issues found! (ran in 2.0s)

Please refer to the following commit for the changes in the at_backupkey_flutter: https://github.com/atsign-foundation/at_widgets/compare/fix_lint_issues_in_backupkey_flutter

- Description for the changelog

• Export "BackupKeyConstants" and "getEncryptedKeys"

[gkc]

Shouldn't AtClientService be marked as @Deprecated?

[sitaram-kalluri]

Shouldn't AtClientService be marked as @Deprecated?

The AtClientService is marked deprecated in at_client_mobile.dart, the changes are merged to trunk. Will also mark the class as deprecated in at_client_service.dart and push the changes.