Bump pointer space only collection.

Add a mode of collection that collects only the bump pointer spaces,
as opposed to the whole heap. It's disabled behind a flag.

TODO: It still scans the entire non-moving space to look for
inter-space references. Use a card table like technique to limit the
scope of scanning and speed it up.

Bug: 11650816
Change-Id: I56cb11e78e47a578bff644e6e6c63d978cfedf39
4 files changed