Add art_build_host_debug to soong config variable

The `ART_BUILD_HOST_DEBUG` environment variable is frequently used in
Android.mk files within the art/ directory. By adding it to
soong_config_variable, we make it accessible to the Soong build system,
which will be essential when converting art modules to Android.bp.

Bug: 347142939
Test: 1) m -j art-libartd-libopenjdkd-host-dependency
      2) ART_BUILD_HOST_DEBUG=false m -j art-libartd-libopenjdkd-host-dependency
      3) cd art && mm/mmm/mmma, then, check `libopenjdkd` be built.
Change-Id: I20401f29a1636b98dd8fc2dc4e679d1832a89435