Update the timer notification string to be clearer based on UX writer.

Bug: b:113854907
Change-Id: I0c2348325f86197b7a930210226798bb3f5df1ed
Reviewed-on: https://chromium-review.googlesource.com/1212099Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Li Lin <llin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590045}

chromeos/chromeos_strings.grd

@@ -128,13 +128,13 @@ Try tapping the mic to ask me anything.
         Nothing found on screen
       </message>
       <message name="IDS_ASSISTANT_TIMER_NOTIFICATION_TITLE" desc="Title for assistant timer notification.">
-        Timer done
+        Time's up
       </message>
       <message name="IDS_ASSISTANT_TIMER_NOTIFICATION_CONTENT" desc="Content for assistant timer notification.">
         00:00s
       </message>
       <message name="IDS_ASSISTANT_STOP_TIMER_QUERY" desc="Query for stopping assistant timer.">
-        stop timer
+        Stop timer
       </message>
     </messages>
   </release>