ANDROID: dm-verity: run in a WQ_HIGHPRI workqueue

(cherry picked from commit a96fcc535ba881b4100eb2c9db8423fe7ef23422)

Running dm-verity in a standard workqueue results in IO competing for
CPU time with standard user apps, which can lead to pipeline bubbles and
seriously degraded performance. Move to a WQ_HIGHPRI workqueue to
protect against that.

Signed-off-by: Tim Murray <timmurray@google.com>
Bug: 25392275
Change-Id: Ic65d7bd6f04e4d77780119e926a50e71323575f0