Revert "Gray only immune objects mutators access."

Fails with:
    art F 11338 11338 art/runtime/gc/collector/concurrent_copying-inl.h:83] Check failed: !kGrayImmuneObject || updated_all_immune_objects_.LoadRelaxed() || gc_grays_immune_objects_ 


Bug: 29516465
Bug: 12687968

This reverts commit 16292fcc.

Change-Id: I1d2d988b7707e03cc94f019cf8bef5b9a9099060