Desktop Experience needs the device to be eligible

The Desktop Experience is only available (and therefore the freeform on
secondary display disabled) if the device is eligible for Desktop Mode.
Correcting the test to make it correct.

Fix: 393547201
Test: atest DesktopModeSecondaryDisplayPreferenceControllerTest
Flag: EXEMPT (bug fix)
Change-Id: I36a031487d8a6f6d681a2cb46a1fa50e47d870cc