Correct handling of desktop experience dev option

Makes sure not only the flag is enabled, but the device is eligible for
desktop mode.

As this is more expensive (e.g. requires getting the application
context, read system properties), move all the logic in
getToggleOverrideFromSystem, including checking for the feature to be
enabled or not, as this differs between the developer options.

Make DesktopExperienceFlags the same.

Flag: EXEMPT (bug fix)
Bug: 389092752
Fix: 414405404
Test: atest DesktopExperienceFlagsTest
Test: atest DesktopModeFlagsTest
Test: atest DesktopModeHelperTest
Test: Manual on various devices
Change-Id: I05b166c1fdd91c6688d7a0b717143a8ac7dfc392