Fix a DCHECK failure due to unmatching numbers of cards scanned.

- See the bug for details of the failure.
- After a discussion, we decided to get rid of the DCHECK as a simple
  solution would not detect corner failure cases and a full solution
  would add undesired complexity, and left a comment that explains
  what situation had caused a DCHECK failure.
- Fix a potential error of failing to scan the last card that the end
  of the image space falls on as a result of the image end being not
  necessarily aligned by the card size.
- Remove dead/unused MarkSweep::ScanRoot().
- Add AlignUp and AlignDown for aligning pointers.

Bug: 11465268
Change-Id: Iee3018a42c48a159feb0e9cf77b1a6b303f5d245