Fix race condition new regions in AllocNonVirtual

Fixed a race condition where a thread could race ahead of the
allocating thread in RegionSpace::AllocNonvirtual and fill the region
before the thread that allocated the region managed to allocate it's
object.

Bug: 63153464
Test: test-art-host
Change-Id: Id0d0e0ac7cdbce6f082234fa82068d8c6c3c7c9e