Allow vndk-sp to be extended by vendors

Exteded vndk-sp should go to /vendor/lib/vndk-sp.

Bug: 37940694
Test: sailfish builds and boots
Test: manually copying a lib from /system/lib/vndk-sp to
/vendor/lib/vndk-sp. Check that the copied one in /vendor partition is
loaded instead of the original one.

Change-Id: Ia5dd6f35bd31d4a20a794bec148e1fc3c6d7b72b