assistant: Request audio focus when voice interaction starts

This cl adds the framework to request audio focus when voice interaction
starts. And releases the audio focus when interaction ends.

Bug: b/119690768
Test: manual
Change-Id: Ic53570a4297df3bd3cbb1e6af76cda3a8cc03a87
Reviewed-on: https://chromium-review.googlesource.com/c/1371014
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618142}