Rewire HoverButton's PressedCallback to support move-only callbacks.

PressedCallback will become non-copyable when it is updated to work with
OnceCallbacks. This causes problems since HoverButton uses its
PressedCallback for the actual underlying button, as well as passing a
copy to the button controller for it to handle additional events.

One potential solution is to use a trampoline RepeatingCallback and
populate HoverButton and HoverButtonController with two different
PressedCallback instances; this idea was discarded because:
- HoverButtonController gates a few behaviours based on whether the
  supplied PressedCallback is null or not, so HoverButton would have to
  conditionally create a trampoline RepeatingCallback only if the
  PressedCallback is not null.
- however, a PressedCallback holding a OnceCallback will become null
  after it is run, and there is no easy way of propagating that null
  state up the PressedCallback chain.

Instead, the solution here is to store the original PressedCallback on
HoverButton itself, and allow HoverButtonController to grab a reference
to the PressedCallback when needed.

HoverButton still needs to pass a trampoline PressedCallback to the
actual underlying button, but this does not have the same issues with
propagating null state up the chain; the underlying button only cares
about the null state to avoid running a null callback.

Bug: 1326449
Change-Id: I37f087c4a43cff942a756b62dc1d5441670bc101
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5069913
Reviewed-by: Elly FJ <ellyjones@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1231412}