[1/n] Exempt dream activities from desktop

Dream activities should always be fullscreen and thus should be exempt
from desktop will all entry points hidden.

Flag: com.android.window.flags.enable_dream_activity_windowing_exclusion
Bug: 414619063
Test: atest WMShellUnitTests:DesktopModeCompatPolicyTest
Change-Id: I810bf97fabcd29830382fe2709f9c100de6535d3