adbd: reduce queue depths.

Each operation we submit results in a contiguous allocation in the
kernel which can trigger OOM-killing. Tune the queue depths to reduce
the chances of this happening, with a minimal impact on performance.

Bug: http://b/126582877
Test: ./benchmark_device.py with blueline
Change-Id: I4b0f83d54a9a9a69f0ab48322d78fb137c94dd35