[1/n] Use fullscreen for modals dialog in desktop mode

When a transparent activity is launched as the only activity of a task
we disable desktop mode and hide the header.

Bug: 319492844
Test: atest WmTests:TaskTests
Test: atest WMShellUnitTests:DesktopTasksControllerTest
Test: atest WMShellUnitTests:DesktopModeWindowDecorViewModelTests
Test: atest WMShellUnitTests:AppCompatUtilsTest

Change-Id: I2ecf7c9d84ffc39fe6f5b28abc5ad7d50b3314a8