Add persistent property to disable strict mode

Setting persist.sys.strictmode.disable to 'true' will disable strict
mode on eng/userdebug builds where it is turned on by default.
Explicitly enabling it in the Settings UI will override this, so
it's still possible to toggle it there even when this property
has been set on the device.

Change-Id: Ifd971f948fb2f803d509c2a06112c4bb932a5b1f