task_profiles.json: Set io.prio.class

Provide an additional way to request the block layer to prioritize
foreground I/O over background I/O. This patch prepares for tests with
the mq-deadline I/O scheduler. While CFQ and BFQ support a "weight"
cgroup attribute, the mq-deadline scheduler does not. The mq-deadline
I/O scheduler only supports the I/O priority class for differentiating
I/O priorities.

The io.prio.class attribute is declared optional since this attribute
only exists if CONFIG_BLK_CGROUP_IOPRIO != n in the kernel
configuration.

Bug: 186902601
Change-Id: Iee1004cd0996e32245aff2b51772ef40178e024f
Signed-off-by: Bart Van Assche <bvanassche@google.com>