sign_virt_apex: use thread pool am: 486609f48f

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Virtualization/+/2067888

Change-Id: Ifaaa870adb154fae0798bc0501f73fcd8d72f3e1
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>