CRAS: cras_shm: rename shm_area to shm_header

In preparation for a split shm:
  rename cras_audio_shm_area to cras_audio_shm_header
  rename cras_audio_shm's field 'area' to 'header'
  update all references to the renamed struct/struct field.

Also slightly clean up shm setup logic for dev_io_stubs and
audio_thread_unittest.

Since this is just a rename of some fields, no functional changes
are expected.

BUG=chromium:968724
TEST=ASAN unit tests, basic on-device playback test

Cq-Depend: chromium:1658535
Change-Id: I0130e9b65add7f84496e8dd37edfa384403203cd
Reviewed-on: https://chromium-review.googlesource.com/1659619
Tested-by: Fletcher Woodruff <fletcherw@chromium.org>
Commit-Ready: Fletcher Woodruff <fletcherw@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Chih-Yang Hsia <paulhsia@chromium.org>
15 files changed