add HotwordTriggered signal

BUG=b:62065175

Change-Id: I004206768850ded54e43263fd0ba61f51878d8fe
Reviewed-on: https://chromium-review.googlesource.com/661947
Commit-Ready: Muyuan Li <muyuanli@chromium.org>
Tested-by: Muyuan Li <muyuanli@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
(cherry picked from commit 8dbf538802c8f50b0ce1f06f579d3a4809592ecc)
Reviewed-on: https://chromium-review.googlesource.com/677093
Commit-Queue: Muyuan Li <muyuanli@chromium.org>
Trybot-Ready: Muyuan Li <muyuanli@chromium.org>
1 file changed