CRAS: Only access dsp context_list from one thread.
CRAS: Only access dsp context_list from one thread.

BUG=chromium:235500
TEST=build and run cras

Change-Id: I105b73ca4f7d4a475bf139bcd9ba510a509dfbeb
Reviewed-on: https://gerrit.chromium.org/gerrit/49286
Reviewed-by: Hsinyu Chao <8b91762bdbe27d3121b311091d7ca5125e9a8df6@chromium.org>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Commit-Queue: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Tested-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

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