ART: Skip image visitor in OatWriter for VerifyAtRuntime

There's no use running the visitor, and it will force-resolve all
methods. Avoid the overhead. Reduces boot image compile time in
verify-at-runtime mode from 1.25s to 1s (on the host, in -j4 mode).

Bug: 24103765
Change-Id: Icdc3be628fed0c9bda01cf5f24fe377f23c571a9