Have the ART Buildbot generate ICU .dat files in /system/etc/icu on target.

Have tools/buildbot-build.sh build target `icu-data-art-test`, copying
ICU .dat prebuilt files into /system/etc/icu, so that they can be
found even if the Runtime APEX is not available, by setting the
environment variable `ART_TEST_ANDROID_RUNTIME_ROOT` to "/system" on
device.

This is a temporary change needed until the ART Buildbot fully
supports the Runtime APEX.

Test: Rely on the ART Buildbot.
Bug: 121117762
Bug: 119293618
Bug: 113373927
Change-Id: Ia1220751df80e47580f6d3c3e65c7169c61b8f3f