ANDROID: dm-crypt: run in a WQ_HIGHPRI workqueue

(cherry pick from commit ad3ac5180979e5dd1f84e4a807f76fb9fb19f814)

Running dm-crypt 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: I589149a31c7b5d322fe2ed5b2476b1f6e3d5ee6f