[Theme] Notify all activities to apply new color theme.

In this CL:
1. Add a singleton class NtpThemeStateProvider and observer interface.
2. ChromeBaseAppCompatActivity registers as an observer of
   NtpThemeStateProvider to get notified when a different color theme
   should apply.
3. The theme button sheet will notify NtpThemeStateProvider when a new
   theme color is selected by users.
Demo: http://shortn/_XXm8sNBZf6

Bug: 423579377
Change-Id: I962013b0ad37a71140b36ceca906b1a1970cd9d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6981869
Reviewed-by: Xinyi Ji <xinyiji@chromium.org>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Commit-Queue: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1520983}