Correct DesktopMode tests to be less dependent on flags.

The tests assume the use of the Settings.Global while for Desktop
Experiences they do not anymore. Instead, we update the cache to avoid
reading the underlying data.

Fix: 396640696
Bug: 389092752
Flag: EXEMPT (Bug fix)
Test: atest DesktopModeStatusTest
Test: atest DesktopModeHelperTest
Change-Id: I1da82d2e8e96355c16a1ddc33489d68e4f92a1be