DeskButton: Fix label discolorations

This CL fixes some issues with contrast between the label and the desk
button background. Previously, the label assumed that we had an opaque
background and so it tried to contrast the text color with this
imaginary background color. We fix this by setting auto color
readability to false for the label. We were also experiencing issues
where sometimes the color for the label text would not update correctly
on theme change. We fixed this by assigning a color ID for the label
rather than a color.

Bug: b/287129850
Change-Id: I77a7779812d9a432cb7cc75cc51f8f93628ae24c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4679620
Reviewed-by: Yongshun Liu <yongshun@chromium.org>
Reviewed-by: Daniel Andersson <dandersson@chromium.org>
Commit-Queue: Ben Becker <benbecker@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1168996}