libz is back to LLNDK

For 2016 pixel devices, where VNDK is not fully enforced, move libz back
to LLNDK so that we can pass the CTS without requiring the default
namespace to be isolated.

If we have libz in vndk-sp directory, test_linker_namespaces fails
because /system/lib/vndk-sp/libz.so becomes accessible. In order to make
the lib inaccessible, we have to make the default namespace isolated,
but this can't be done for 2016 pixel devices where VNDK is not fully
enforced. So, we choose to remove /system/lib/vndk-sp/libz.so and keep
the single copy at /system/lib.

Bug: 65018779
Test: android.jni.cts.JniStaticTest#test_linker_namespaces successful on
2016 pixel devices

Change-Id: Ib6b6560b02be69d2015c0afb86acf07c02b30c2f