Add support for test_apex

Some test and other miscallaneous use-cases need apexs that are
installable but will not affect the installation of other artifacts
onto the device. For this purpose we added a test_apex type. These
test_apex targets create apex files and have all the normal attributes
but are not considered the source of any of their contents for the
device by the rest of the build-system.

Test: build and boot
Test: ALLOW_MISSING_DEPENDENCIES=true \
      DIST_DIR=$ANDROID_BUILD_TOP/out/dist \
      ./art/tools/dist_linux_bionic.sh -j72 showcommands com.android.runtime.host
Bug: 123591866
Bug: 123892969

Change-Id: I7f50be0ac0425cd87868145e18bcee6962d472ef