Revert "Move libz to vndk-sp"

This commit removes libz.so from the exported libs of "vndk" linker
namespace and export libz.so from "default" linker namespace instead.

We have to remove libz.so from vndk-sp because we don't have a good
mechanism to stop "classloader" linker namespace from loading
/system/lib[64]/vndk-sp/libz.so.  Other alternatives require significant
changes to dynamic linker or CTS test cases.

Let's temporarily revert the commit to fix b/62195853 for oc-dr1-dev.
The long-term solution will be discussed in b/37617391.

This reverts commit 63226400.

Bug: 62195853
Bug: 37617391
Test: Boot sailfish to home screen.
Change-Id: I4a97819b080f7091b633152d4a11353b24c463ee