[PM-4327] Adjust LaunchMode of WebAuth & Autofill helper activities

[mpbw2]

Type of change

• [X] Bug fix
• [ ] New feature development
• [ ] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
• [ ] Build/deploy pipeline (DevOps)
• [ ] Other

Objective

Updated the LaunchMode of helper activities to restrict access in older versions of Android

Code changes

• AutofillExternalSelectionActivity.cs: Change launch mode to SingleInstance and add explicit export declaration
• WebAuthCallbackActivity.cs: Change launch mode to SingleInstance

Before you submit

• Please check for formatting errors (dotnet format --verify-no-changes) (required)
• Please add unit tests where it makes sense to do so (encouraged but not required)
• If this change requires a documentation update - notify the documentation team
• If this change has particular deployment requirements - notify the DevOps team

[bitwarden-bot]

Checkmarx One – Scan Summary & Details – fb98df5b-6149-44f4-bfdb-8f0a7cba5b7d

No New Or Fixed Issues Found

[thBGQIXdam4xJyYT1NoeyWaaY2V32AT4G9C4ayo]

Checkmarx One – Scan Summary & Details – fb98df5b-6149-44f4-bfdb-8f0a7cba5b7d

No New Or Fixed Issues Found