Reduce allocations in ParallelGC test.
Reduce allocations in ParallelGC test.

Fixes GC stress test mode.

Bug: 16406852
Change-Id: I86c8fd7516c3f41075bca00c7d5b4dab2bb7b1d4

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst