Clean up libzipfile Android.mk

Switch to using libz instead of libunz as per the note in
external/zlib/Android.mk.

Removed LOCAL_C_INCLUDES which are no longer necessary as zlib
now uses the LOCAL_EXPORT_C_INCLUDE_DIRS mechanism.

Tested compiling the projects that make use of libzipfile using
a clean out dir with both aosp_flounder-userdebug and aosp_arm-eng

make -j16 test_zipfile fastboot adb adbd libSR_Core libsrec_jni

Change-Id: I5fdf34b6c272ae64c406e5b77ab67a5f5e4b5890