Bring back isDeviceEligibleForDesktopMode, couple it with canHostDesktop

This brings back the old logic of dictating whether a device can have be
eligible for desktop mode or not, either via dev options or via actual
support. However the actual support now relies on two configs:
desktopModeSupported and canInternalDisplayHostDesktops.

Bug: 395826065
Test: Manually check on tangor devices, which currently have desktop mode disabled by default
Flag: EXEMPT bugfix
Change-Id: Id6c177bd06217eba856195a55a636ffb86e33742