Move allocation stack unmarking outside of pause.

Reduces pause time by moving the allocation stack unmarking outside
of the pause. This is especially helpful for devices which have longer
GC times since these times result in having more things to be
unmarked in the allocation stack.

Bug: 9969166

Change-Id: I570f2213cbdda9d90545b64538e2cbeb0dc32d16
4 files changed