Gray only immune objects mutators access.

To reduce image/zygote dirty pages.

GC doesn't gray immune space objects except when visiting the thread
GC roots of suspended threads during the thread flip. GC updates the
fields of immune space objects without pushing/popping them through
the mark stack. GC sets a bool flag after updating the fields of
immune space objects. After this point, mutators don't need gray to
immune space objects. Removed the mark bitmaps for immune spaces.

Bug: 29516465
Bug: 12687968
Test: art tests, libartd device boot, ritzperf.
Change-Id: Idfcffbdb94dfc8bfc89c30d6aff8888f04990a56