Add emoji bar to PickerView to show emojis, symbols and emoticons.

- Add PickerEmojiBarView as a "floating UI" above the main Picker
  container.
- Show expression results (emojis, symbols, emoticons) in this emoji bar
  instead of the main Picker container.
- Add basic pseudo focus handling so that users can use keyboard to
  navigate between the emoji bar and main Picker container.

Since "pseudo focus" was initially designed with the assumption that the
main container would be the only Picker container, it's currently
slightly buggy (e.g. PickerView doesn't "centrally" track the pseudo
focused view, so race conditions can occasionally make both the emoji
bar and main container appear pseudo focused, even if only one is
actually responding to user events). We can fix remaining bugs in follow
ups.

Bug: b:340692585, b:340692819
Change-Id: I47887307828ae67882bd35282ee4bed9139f6739
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5599863


Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Michelle Chen <michellegc@google.com>
Cr-Commit-Position: refs/heads/main@{#1311054}