add wallpaper search processing time metrics

1. time in ms since we receive results until background pref is set
2. time in ms from handler call for setting history theme until
   background pref is set
3. time in ms from handler call for setting search result theme until
   background pref is set

Bug: b/308355660
Change-Id: I2c21effa0cee89bb4400058558390f8975aae788
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5078815
Commit-Queue: Riley Tatum <rtatum@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1232939}