Move ImageMutator after archMutator

Move the ImageMutator to be registered just after the archMutator
in preparation for moving it between osMutator and archMutator.
Requries updating variants in a few tests that now run the
ImageMutator.

Bug: 142286466
Test: no change to build.ninja
Test: all soong tests
Change-Id: Ia9d2a7bc0e225bedec3c9a83ea04f471a931bf47