shortcuts: Add Edit-button for editable accelerator.

* Add edit icon at the end of accelerator-row if the accelerator is
  editable and clicking edit-icon will open the edit-dialog.
* Update and add unit tests for this behavior.
* video: http://go/scrcast/NTU3NDIyNzEyNzE3MzEyMHxmMGM1ZTFhMy02OQ

Bug: b/216049298
Test: browser_tests --gtest_filter=ShortcutCustomizationApp*
Change-Id: Id4eada5cf30979c755cf1af926d632fc6294b840
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4539941


Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Commit-Queue: Longbo Wei <longbowei@google.com>
Cr-Commit-Position: refs/heads/main@{#1146739}