[2/n] Include platform signed apps in transparent windowing exemption policy

Flag: com.android.window.flags.enable_modals_fullscreen_with_platform_signature
Test: atest WMShellUnitTests:DesktopModeCompatPolicyTest,
      atest WMShellUnitTests:DesktopTasksControllerTest
Fixes: 407759199
Change-Id: If43008f3c96ab8b81e436835f7645a316f8f4928