Add support for option `--android-runtime-root` to ART run-test scripts.

Also have testrunner.py honor `ART_TEST_ANDROID_RUNTIME_ROOT` and pass
its value via `--android-runtime-root` to art/test/run-test (for
target testing).

These changes are needed for the interim period where
- we have code running on the ART Buildbot using
  `ANDROID_RUNTIME_ROOT` to locate files on device (namely ICU code
  looking for ICU .dat prebuilt files); and
- the Runtime APEX is not (yet) available on all environments where
  ART is running (namely the ART Buildbot).

Test: Rely on the ART Buildbot.
Bug: 121117762
Bug: 119293618
Bug: 113373927
Change-Id: I83bbb2442f15ed82eee3369dad1d249227bb3442