dex_import that isn't available for platform isn't installed

This change fixes a bug that dex_import module is always installed to
the platform even when the module is not available to the platform.

Bug: 158284983
Test: OUT_DIR=/tmp/ndk build/soong/scripts/build-ndk-prebuilts.sh
Change-Id: I85570506e700be59ee63e4f1d7902a40e36df39c