Move libz to vndk-sp

This commit moves libz.so from default namespace to vndk namespace.
Accodring to b/37617391, libz.so is not as stable as we thought, thus
it should be vndk-sp instead of ll-ndk.

Bug: 37617391
Test: sailfish builds, boots, no new dynamic linker errors in log
Change-Id: I3922895a169303c33914a8a0f4b50cb993a724c9