Change the region space region size to 256k.

Change the region space region size to 256k.

Also add RegionSpace::non_free_region_index_limit_ to avoid the need
to scan the free end of the region table in SetFromSpace(), which
compensates (and more) the pause time increase due to the increasing
number of regions.

Ritz EAAC avg pause time (angler little core / -Xmx512m):
Before 186us
After  436us (without non_free_region_index_limit_)
After  103us

Partially revert aog/327342 and remove the temporary adjustment of
max/min-free. Changing the region size to 256k was enough to avoid the
GCE boot issue (b/34576638), but 154-gc-loop barely fails. Make
154-gc-loop failures less strict.

Allocation performance (angler little core / -Xmx512m)
Ritz EAAC
Before 939.6
After 937.8

BinaryTree
Before 603350
After 620200 (-3%)

Bug: 12687968
Test: test-art-host
Test: GCE boot
Change-Id: I1495ab4ced806e1c4d779d49b56cea618817a0d6