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This event and corresponding interface will be used to support battery
indicator for headphone supporting Hands-Free Profile (HFP).

Design doc: go/cros-bt-bi-hfp

BUG=chromium:785758,b:140783217
TEST=build chrome and run chromeos_unittests

Change-Id: Ieda4b83f4aa86f21038e7e6a9e9e5aa2b58cba0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2041970
Auto-Submit: En-Shuo Hsu <enshuo@chromium.org>
Commit-Queue: En-Shuo Hsu <enshuo@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740851}