[Dark] Listen to system UI mode

Create a class to listen to system UI mode changes (i.e application
context UI mode), and make both app and custom tab night mode
controller observe system UI mode changes.

Bug: 942769
Change-Id: I814b51e65047025f6e9087bb482c4ae7b0e76b91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1526697
Commit-Queue: Becky Zhou <huayinz@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642733}