[omnibox][history-embeddings] Hide loading answers after `Stop()`

The history embeddings answers are allowed to ignore inactivity
`Stop()`s because:
1) Answers are expected to take longer to load than the 1.5s stop timer.
2) Answers are sorted 2nd-to-last above the disclaimer IPH, and changes
   lower in the popup are less disruptive than changes higher up.
3) Answers reserve their slot with a 'loading' placeholder, and
   replacing the 'loading' placeholder with an answer is less disruptive
   than showing a brand new match.

Allowing this stop-exception is delicate. There are 3 `Stop()` scenarios
we need handle:

1) Inactivity `Stop()`
  a) User types '@history query'
  b) 'loading' answer is displayed
  c) 1.5s later the stop timeout triggers causing `Stop()` to be called
  d) when the real answer is ready, it replaces the 'loading' answer

2) Popup closure `Stop()`
  a) User types '@history query'
  b) 'loading' answer is displayed
  c) user closes the omnibox causing `Stop()` to be called
  d) when the real answer is ready, it should be discarded and not
     re-open the closed popup

3) Other `Stop()`
  a) User types '@history query'
  b) 'loading' answer is displayed
  c) user presses the down arrow causing `Stop()` to be called
  d) the 'loading' placeholder is removed
  e) when the real answer is ready, it shouldn't update the popup
     disrupting the user's selection

With this CL, all 3 cases work correctly. But it's still a temporary
solution; see crrev.com/c/5948144 & crrev.com/c/5980091 .

Bug: 364303536, 376329281
Change-Id: I8e1fc96858df999715e636d2f60805f5a3099c47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5998150
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Auto-Submit: manuk hovanesian <manukh@chromium.org>
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Reviewed-by: Orin Jaworski <orinj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1379248}