sign_virt_apex: use thread pool am: 486609f48f am: 1f4a8a86b3 am: f291de7131 am: 7ccb02be6e

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

Change-Id: I0c9630fdc30c723cc78cc70aca58471c90291e3c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>