Simplify DesktopModeFlags

If the Desktop Experience developer option flag is enabled,
DesktopModeFlags will look at the same signal as DesktopExperienceFlags
(i.e. the system property).

If not, it will look at the previous signal (i.e. the settings config).

This will simplify the code and avoid some expensive blocking calls.

Fix: 416813838
Bug: 389092752
Test: atest DesktopModeStatusTest DesktopStateImplTest
Test: atest DesktopExperienceFlagsTest DesktopModeFlagsTest
Flag: EXEMPT (bug fix)
Change-Id: I2e5753202f38de2f39d475fcf50c7fffb1fc4849