assistant: fix crash in OnTtsStarted

Some times |OnTtsStarted| will be called multiple times during a single
interction session. This break the previous assumption that Tts is always
the last response and causes NPE.

Bug: 926303
Test: locally build and test, cannot repro crash with change.
Change-Id: Ib97f1ca188236af903dde1c6aa7d6e6c7d6f7b50
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1531304
Reviewed-by: Yue Li <updowndota@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642632}