Clear (madvise) card table for CC

Clear (madvise) card table for CC

Since the cards are not really used, we can madvise them to reduce
RAM. Image and zygote cards are cleared in pause to prevent races
between with mutators.

Pause time goes up by only 1.5us on N6P maps:
(Paused)ClearCards: Sum: 755us 99% C.I. 0.250us-28us Avg: 1.540us Max: 28us

AOSP N6P before (60s after boot system wide CC):
4,194K: .GC
4,172K: .GC
4,110K: .GC

After:
3,253K: .GC
3,205K: .GC
3,245K: .GC

Bug: 12687968

Test: test-art-host

Change-Id: I3194b5b8044c2dca427302c32d9974920fecb289