Fix padding computation in ArenaAllocator::AllocAlign16() am: 68353fecf7

Original change: https://android-review.googlesource.com/c/platform/art/+/2202585

Change-Id: Idc74eb92a8d92fdf970b6cc9e895f921592fa5ca
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>