Porting over iOS fix for self-hosted SSO to Android from #11563.
Fixes#12819
Tested on own instance using Authentik.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Android authentication now supports an optional server parameter in
the callback URL, enabling sign-in against different server instances.
* If the specified server cannot be found, the authentication attempt is
halted and an error is reported.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
---------
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>
