Change com.android.runtime.host to be an installable test_apex

We need this module to actually be built and installed to the
out-directory but we don't want it to affect the libraries installed
on actual devices so make it a test_apex.

Bug: 123892969
Bug: 123892969
Test: Build and boot
Test: ./art/test/testrunner/run_build_test_target.py -j72 art-linux-bionic-x64-zipapex
Change-Id: I9950698863fb280cbdc6dfa4743985c0217d2e7c