sign_virt_apex: use thread pool am: 486609f48f am: 1f4a8a86b3

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

Change-Id: I6e8dad9186c412057b70a34ad1e5e8915d8852a8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>