vndk-def: Add libz.so back to LL-NDK

This commit adds libz.so back to LL-NDK because the change to move
libz.so to VNDK-SP breaks some CTS testcases (which assume there is only
one libz.so under /system/lib[64].)

Whether this is a short-term temporary fix or a long-term solution will
be discussed or updated in b/37617391.

Bug: 62195853
Bug: 37867089
Bug: 37617391

Test: ./tests/test_ndk_libs.py
Test: ./tests/run.py
Test: libz.so tagged as LL-NDK with sailfish images.

Change-Id: I526f653e6141d658ee19e79eefd44d379ecac0dc