Have testrunner honor ART_TEST_CHROOT only for target tests.

Chroot-based testing is only supported on devices, and
art/test/run-test would complain if `--host` and `--chroot` were to be
used together.

Also have the same behavior for ART_TEST_ANDROID_ROOT, as it only
makes sense for target tests.

Test: ART_TEST_CHROOT=/data/local/art-test-chroot art/test/testrunner/testrunner.py --host
Bug: 34729697
Change-Id: I617fbcf719d3c59dbcbd632e69bf52e3498b4cff