Fix offline release build failing on desugar_jdk_libs (CI failure blocking #718)

[kasnder]

Summary

The test job's ./gradlew :app:assembleFdroidRelease --offline step fails on CI (currently blocking #718 and any other PR, unrelated to their own diffs):

Could not resolve all files for configuration ':app:coreLibraryDesugaring'.
> Failed to transform desugar_jdk_libs-2.1.5.jar (com.android.tools:desugar_jdk_libs:2.1.5) ...
  > Could not download desugar_jdk_libs-2.1.5.jar (com.android.tools:desugar_jdk_libs:2.1.5): No cached version available for offline mode

prefetchOfflineBuildClasspaths (app/build.gradle) resolves the runtime/compile classpaths needed for the later --offline steps, but never resolved the coreLibraryDesugaring configuration, so desugar_jdk_libs was never downloaded while networking was available.

• Add coreLibraryDesugaring to the list of configurations prefetchOfflineBuildClasspaths resolves.

Test plan

• app/build.gradle parses (Gradle configuration phase reaches the SDK-location check, i.e. no Groovy/DSL errors) — could not run further, since this sandbox has no Android SDK installed.
• CI green on this PR (test job's offline assembleFdroidRelease step)

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