[PDF Ink Signatures] Refactor alignment to a custom element

- Refactor the text alignment controls to a new custom element.
- Add a mixin for observing text changes, and use it from the new
  control as well as the text side panel and the style selector.
- Refactor test for alignment controls.
- Update styles to match 16px spacing in mock and to enable the
  alignment to be also shown from a viewer-bottom-toolbar-dropdown.

This is in preparation for adding a bottom toolbar, which will also
require an alignment selector.

Bug: 402546154
Change-Id: I37caac8e2bd11a263b7edf102cb257c4dbc8106c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6421503
Reviewed-by: Alan Screen <awscreen@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1441907}