Have Golem generate ICU .dat files in /system/etc/icu on target.

Have Golem target build rule (`build-art-target-golem`) 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 "$ANDROID_ROOT" (i.e. "/system") in
the `art` script on device.

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

Test: Rely on Golem.
Bug: 121117762
Bug: 119293618
Bug: 113373927
Change-Id: I0342e863eb2354b277b56f346c71e0624b556dec