Make Picker feature tour widget activatable, to support keyboard events.

Make the Picker feature tour dialog widget activatable, so that it gains
focus when shown and can receive keyboard events. These keyboard events
are then handled with default SystemDialogDelegateView behaviour.

This change means that the Picker feature tour widget steals focus from
the initially focused input field if there was one. So, we also need to
ensure that focus is returned after completing the feature tour and
before opening the Picker widget (otherwise the Picker widget may appear
in the wrong position and in the wrong state).

To do this, create a PickerActionOnNextFocusRequest to show the Picker
widget only after focus is regained (if needed). In case focus somehow
fails to be regained, also add a timeout after which we stop waiting for
focus and show the Picker widget regardless of the focus state.

Bug: b:353730051
Change-Id: I225dd8d73be675da3e75346122b58c75792e95d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5743234
Commit-Queue: Michelle Chen <michellegc@google.com>
Reviewed-by: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1334060}