[PDF Ink Signatures] Add brush color options in side panel UI

Add color options to the annotation mode side panel UI so that the user
can select the brush color.

When a user chooses between brushes, the color setting should be saved
in the current session. When the user goes back to a brush they've
already used, it will be the same color that the user last set.
Additionally, the color option should be shown as "checked".

Style the color buttons similar to ChromeOS Gallery, with radio buttons
that grow larger when checked. The styling isn't exact and can be
adjusted in the future.

Bug: 351868764
Change-Id: Icb5761b81acb2ee926317f5e1c4eda72146b52f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5710988
Commit-Queue: Andy Phan <andyphan@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1329197}