Define `ANDROID_RUNTIME_ROOT` as `$ANDROID_ROOT` on target in the `art` script.

This is a temporary workaround meant to help ICU find ICU .dat files
in "/system/icu/etc" on target, until the Runtime APEX is available in
target testing on the ART Buildbot.

On host, continue to define `ANDROID_RUNTIME_ROOT` as
"$ANDROID_ROOT/com.android.apex".

Test: Rely on the ART Buildbot.
Bug: 121117762
Bug: 119293618
Bug: 113373927

Change-Id: Ie5e4448d2ec12e2ee499b66811fce5a076bb0bf4