Add a quick SELinux mode toggle

There's no way to make it persistent, by design. This is purposefully
undocumented and meant to test policies at this stage.

Change-Id: Ic79ed773b199b68fee15f94acd2107a5ad20dc25
Merge "Add a quick SELinux mode toggle" into cm-10.1
Merge branch 'cm-10.1' of git://github.com/CyanogenMod/android_packages_apps_Settings into cm-10.1-n7