Revert "Revert "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 and in
FillWithDummyObject(). 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.

This reverts commit ddeb172e.

Bug: 29516465
Bug: 12687968
Test: art tests, libartd device boot, ritzperf, jdwp test.
Change-Id: If272373aea3d41b2719e40a6a41f44d9299ba309