[zip] Move the singleton instance function for inlining am: 586c6d4f81 am: 416507a524

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

Change-Id: Ia72fb648f3f6957e4bbfffeec34e24ece86e8ff2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>