Persistent bitmap for region space

Fix bug where region space was not safely walkable due to holes
in the unevac regions possibly having dangling class pointers.

No preformance change, RAM overhead 1.1-1.2% .Heap (non LOS).

Test: test-art-host
Test: https://android-review.googlesource.com/#/c/288907/

Bug: 31522820

Change-Id: Ic4f8b7175e117689cb1ce3e28b082cf63f1f7b5a