Revert^2 "Re-enable ART's region space memory protection on host."

Memory protection of unused regions was disabled (again) on host
because of spurious errors due to old kernel issues on some
machines. These problems are presumably fixed now, so let's
re-enable this feature.

This reverts commit 8e304109.

Reason for revert: The hosts that exhibited spurious SEGV_ACCERRs
have had their kernels updated from Linux 3.13 to Linux 4.4.

Test: Rely on the ART Buildbot
Bug: 63131961
Bug: 74064045
Bug: 116841931
Bug: crbug.com/883241
Change-Id: I75be818e4c370e1364b89379d1e759b507e256e3