[webview] Add a force dark WebSetting

Force dark mode is a tristate:

- never invert colours for webview content
- always invert colours
- follow the parent view behaviour

Test: CTS test, after WebView implementation.
Bug: 120599879
Change-Id: Ib11358bc3a3cbc5e55c0ec184084c7d205acf60c
2 files changed