Clear zygote space cards for CC

There may be dirty cards from reference processing or from zygote
compaction. Having these dirty cards in the zygote space is
unnecessary and causes PSS increase from GrayAllDirtyImmuneObjects
in future GCs. Clearing the zygote space cards when the zygote
space is created fixes this issue and reduces PSS.

System wide PSS for zygote space, before:
7,899K: .Zygote
7,896K: .Zygote
8,067K: .Zygote
8,547K: .Zygote

After:
4,881K: .Zygote
5,003K: .Zygote
5,582K: .Zygote
5,259K: .Zygote
5,176K: .Zygote

Bug: 12687968
Bug: 37219911

Test: Device boots
Test: test-art-host

Change-Id: I92daacef51c13ba7536fa18e94af67674d13089f