Bump external/xamarin-android-tools from 05f9a90 to c8a5b5b (#1216)

Bumps [external/xamarin-android-tools](https://github.com/xamarin/xamarin-android-tools) from `05f9a90` to `c8a5b5b`.
- [Commits](dotnet/android-tools@05f9a90...c8a5b5b)

---
updated-dependencies:
- dependency-name: external/xamarin-android-tools
  dependency-type: direct:production
...

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