[2/n] Handle permission for transparent activities

Integrates logic in DesktopModeCompatPolicy to allow only apps
with SYSTEM_ALERT_WINDOW permission to reach fully
transparent fullscreen.

Flag: com.android.window.flags.enable_modals_fullscreen_with_permission
Bug: 394714626
Test: atest WMShellUnitTests:DesktopModeCompatPolicyTest

Change-Id: Ie47bb6b412e94dd842914517c37f1c63d8ee69c3