Add AppConfig Support for MDM Managed Configurations

[ManfredWisniewski]

AppConfig Support

• AppConfigManager now exposes managed username and app-password keys alongside the server URL for reuse by dependent
• ServerSelectionActivity pre-fills the managed base URL on first launch and, after capability checks, bypasses browser login when managed credentials are present, handing them directly to account
• Declared the username and app-password restrictions in app_restrictions.xml so MDM providers can supply them via managed configuration.

🚧 TODO

• Also implement these changes for the Nextcloud App

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
  Manual Testing of the MDM support was done with Samsung KNOX Manage platform.
• 🔖 Capability is checked or not needed
• 🔙 Backport requests are created or not needed: /backport to stable-xx.x
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI
  Changes proposed in this PR were created with the help of GPT-5-Codex.
  Building and testing the App in Android Studio was supported by Gemini.

[mahibi]

Hi @ManfredWisniewski
thank you for contributing. We will have a closer look at the Pull request soon.