Fix padding computation in ArenaAllocator::AllocAlign16() am: 68353fecf7 am: bd71a080f0 am: ad36d51b3a am: bb7565924e am: a06b6828da

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

Change-Id: Ibb208ffd1ec096b5360ca83a4476bdfc1932f885
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>