Use the card table to scan the immune region of the heap.

Use the card table to scan the immune region of the heap.

Using the card table results in a dramatic cost reduction in scanning
time.  During a reboot, scanning each object in the zygote for
pointers to the application heap costs an average of 20ms of thread
time on a stingray with 8 megabyte zygote.  In comparison, scanning
dirty cards in the zygote costs 300us on average.

Change-Id: I1dba35646d509e6b1b4535e291a1eb6f66d7b218