ART: Replace or remove some ObjectCallback Walk functions

Replace with visitor functions in RegionSpace and SpaceBitmap. Remove
old ObjectCallback version in HeapBitmap. Fix up users. Move some
thread-safety annotations.

Move ObjectCallback definition to the only remaining user (ModUnionTable).

Test: m
Change-Id: I10307aeacad0c60d21fbade2081ec040d6a6ac4c