Disable adding main and non moving spaces to immune region in GSS

Disabled adding the main and non moving space to the immune region.
This will enable us to recycle bump pointer spaces for malloc space
-> malloc space compaction as well as collector transitions.

Also added logic for falling back to the non moving space, we may
copy objects there.

Refactored mod union table logic into MarkReachableObjects.

No measurable performance benefit or regression.

Bug: 14059466
Bug: 16291259

Change-Id: If663d9fdbde943b988173b7f6ac844e5f78a0327