Redo AmbientControllerTest.ShouldDismissScreenSaverPreviewOnTouch.

It fails when TimeOfDay is default enabled, but it's not really
because of TimeOfDay. It comes down to the fact that the video
screen saver can start rendering immediately, whereas the slide
show screen saver must wait for photos to be downloaded. The test
case was not waiting for the ambient ui to start rendering:
* Ambient session is started
* PressTouch
* Ambient session is closed
* Ambient session starts again somehow
* ReleaseTouch
* Ambient session closes again

This sequence is unrealistic. It requires a user to exit ambient
mode by touching the screen, hold their finger on
the screen the entire time, and while doing so, somehow relaunch
ambient mode, then release their finger afterwards. Even if that
somehow happens, it's not clear that the intended behavior is for
the release to trigger ambient session closing again. It does not
work for video because this happens instead:
* Ambient session is launched
* Ambient widget/window is created
* PressTouch
* Ambient widget/window is closed
* Ambient session is closed
* A TOUCH_CANCELED is automatically generated since the PressTouch
  closed the widget while the widget was still active.
* Ambient session starts again somehow.
* ReleaseTouch.
* The GestureRecognizer deep in the ui code does not publish
  the ReleaseTouch to the AmbientController because the original
  PressTouch event was canceled previously when the widget was
  destroyed.

The fix is to give the test a realistic sequence of events and
make sure that touching the screen exits ambient mode while it
is preparing photos and while it is rendering.

Bug: b:292294050
Change-Id: Idb696016a8a4864232fc95d9039435cc69023d61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4713674
Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Eric Sum <esum@google.com>
Cr-Commit-Position: refs/heads/main@{#1175151}