Always mark zygote large objects for CC

Always mark zygote large objects for CC

Prevent needing to gray holders of zygote large objects.
System wide zygote space PSS after boot:
12644 kB -> 5571 kB for CC.

Also PSS reduction in zygote large objects themselves since their
gray bit would have been set each GC.
Overall LOS savings hard to measure, could be up to 316 * 4KB per
app since there are 316 zygote large objects.

Also clear mod-union tables for image spaces to prevent dirty
image pages if any of the image spaces point to zygote large objects.
System wide .art mmap: 37432 kB -> 34372 kB

System server before (N6P):
LOS shared dirty: 12888 kB
Zygote space shared dirty: 700 kB
Zygote space private dirty: 868 kB
.art private dirty: 1696 kB

After:
LOS shared dirty 13672 kB
Zygote space shared dirty: 1072 kB
Zygote space private dirty: 496 kB
.art private dirty: 1432 kB

Bug: 29516968

Test: test-art-host with baker CC, debug N6P phone booting

Change-Id: Ia37ce2c11217cf56885bd1d1dc084332fcbb7843