[zip] Move the singleton instance function for inlining am: 586c6d4f81

Original change: https://android-review.googlesource.com/c/platform/system/libziparchive/+/3426665

Change-Id: Ie10f5bc84af711607ed5d2ab8ac2e3e5a75c0e8a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>