Cleanup vroot test and fix false positive.

The previous test was too complicated. Use the kernel binary
sysctl to exercise the get_user call instead. Besides being
simpler, it has the advantage of not crashing the device
if the vulnerability is present.

Credit for the idea comes from https://twitter.com/grsecurity/status/401443359912239105

Bug: 15408470
Change-Id: Ia95191b4ebbf9310d7ddcf499a106b0561e10dea