Always build zipapex's unflattened.

Zipapex's cannot be flattened so do not prevent them from being built
even if TARGET_FLATTEN_APEX is set.

Test: lunch aosp_arm-eng;
      ALLOW_MISSING_DEPENDENCIES=true \
      ./art/tools/build_linux_bionic.sh com.android.runtime.host

Bug: 124333446
Change-Id: I4bef65cce100136a8f7852692e841b62ed9e399f