arc: Stop stale arc-prepare-host-generated-dir job

When chrome is restarted (e.g. because of crash or being restarted by a tast test), there may be a arc-prepare-host-generated-dir job running.
The stale job can cause an AlreadyStarted error when the restarted chrome tries to start a new job, and results in ARC not starting.
To prevent this, stop the stale job if any.

BUG=b:244687552
TEST=tast run arc.Boot.vm
TEST=unit_tests

Change-Id: I2f61ff356a42a658ed31fb2260d16dfc066029db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3881868
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Commit-Queue: Ryo Hashimoto <hashimoto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1046249}