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

(cherrypicked from db7aafb5)

Bug: 15408470
Change-Id: I25137913f0671037b2e24101bf0097f2c2fadee6