Use DesktopExperienceFlag for KQS flatenning flag

- The KQS flatenning flag is defined in Launcher3 aconfig lib. So use
  the DesktopExperienceFlag class helper method to use the flag through
  developer option.

Test: m
Bug: 382769617
Flag: EXEMPT updates to use existing flag in a different way
Change-Id: I9d4f025f7a9ff4ac6aba008b0763a824bc9f72ba