Add hidl-gen to build tools

Add hidl-gen.  It won't be used by the build, but it will be
convenient to have a prebuilt for some unbundled use cases.

Bug: 165936459
Test: OUT_DIR=out prebuilts/build-tools/build-prebuilts.sh
Change-Id: Ie6de91530238430bd0a86a12982a83769969d1d6