Remove DesktopTaskView.isDesktopModeSupported

- Use Flags.eanbleDesktopWindowingMode directly
- Also clean up unnecessary and improper flag usage

Bug: 309008406
Flag: ACONFIG com.android.window.flags.enable_desktop_windowing_mode DEVELOPMENT
Test: presubmit
Change-Id: I86ce317ce577f279d2ce819d6dc1738204c2631e
22 files changed