Add a new enum for desktop experience flags

This has to be different from desktop mode flags, to allow enabling them independently.

Bug: 389092752
Change-Id: I2b92537cf15fd392ca0c60fe073b7f3b5cad78e9
Flag: com.android.window.flags.show_desktop_experience_dev_option
Test: atest DesktopExperienceFlagsTest