Quicker partial collection by using card marking.

Quicker partial collection by using card marking.

Add calls to the card marking from the write barrier routines, so that
a write to an Object marks the appropriate card. Add code in the GC to
use and rebuild the cards at a partial GC, clearing cards in the
Zygote heap which do not in fact contain references to the application
heap.

Change-Id: Ie6f29fd096e029f48085715b282b6db8a7122555