Add a libz_current static lib for NDK builds.

Using libz via the normal NDK mechanisms means that old headers are
exposed to the build since the NDK libz header is the minimum
supported version (Jelly Bean). Expose a separate libz_current for
platform built `sdk_version: "current"` modules that statically link
libz.

Test: mm
Bug: None
Change-Id: Ieaf3f991958bac8ed637e1f8707eb7dfedd8a5a6