Add support for disabling the greylist.

Useful for testing whether apps have actually stopped using greylisted
libraries even if they still have references to them in their apk to support
old Android releases but also haven't bumped their targetSdkVersion yet.

Since we already have two expensive __system_property_get calls and this
would add a third, optimize two (but leave the third since it's not
obviously amenable to optimization). None of this matters for user builds,
but I don't want userdebug/eng to have distractingly different performance.

(cherrypick of 7933bec2.)

Bug: http://b/36106661
Test: ran "can you escape 5" with and without this property
Change-Id: Id9a804695c1dca9b4be2ebd0e72f01817bb13cba