Bug 1767172 - Remove custom screen info handling from GfxInfo, use ScreenManager for that. r=jrmuizel,handyman,stransky

This removes a bunch of custom code from GfxInfo to obtain screen
information, and instead collects that screen information in
ScreenManager.

This, apart of removing duplicated code, has the extra benefit of
reporting multi-monitor information on GTK (and potentially in the
future reporting scale and refresh rate properly as well, I've kept the
telemetry as it was on that regard).

Differential Revision: https://phabricator.services.mozilla.com/D145178