Add help bubble and ping to holding space tour.

When the user drags a file over the wallpaper, show a help bubble over
the holding space tray and ping it to draw the user's attention.

Still TODO:
* Rate limit help bubble
* Finalize strings
* Explicitly close help bubble when the user opens holding space or
  successfully pins a file to holding space

Demo: http://shortn/_9ZlW452Ehw

Bug: b:283172403
Change-Id: Id7356e1fcc07c99dffbc012eb1ada1eecf2da12e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4546161


Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Colin Kincaid <ckincaid@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147377}