Scan immune spaces using mod union tables.

It's faster to scan the immune spaces using the mod union tables (gray
objects on dirty pages) than the live bitmaps (gray objects).

The immune space scan time goes from ~13ms down to ~10ms in the
testWidgetsContainerFling test on angler at 1 GHz.

Change-Id: I0914d7bdb50cbf8a49c859070a3501b48eb79f3e
Test: N9 boot, art tests, Ritz EAAC.
Bug: 29516968
Bug: 29517059
Bug: 12687968