Avoid unnecessary read barriers in `ImageWriter`.

Also avoid visiting native roots during the same stages,
namely layout and writing (including copy-and-fixup).

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 181943478
Change-Id: I0d62d67556109dbcbd8a04fd0e8408733dacce33
14 files changed