Bug 1754858 - Simplify screen orientation API implementation. r=smaug,m_kato,geckoview-reviewers

Make the ScreenOrientation part of the screen struct, as it should. Stop
using HAL to propagate just screen orientation updates, use the more
general screen manager.

Instead of HAL observers, add a simple observer service notification,
and clean a bunch of the code.

This will simplify bug 1754802 a bit, and is generally simpler.
Shouldn't change behavior. I've tested the events and some common
orientation locking use cases like Youtube, and they behave the same.

Differential Revision: https://phabricator.services.mozilla.com/D138477