Make testAddingImmersiveWindow robust when remote inset controller is used.

Using a sub window type allows the test to be valid even when remote
inset controller is used.

Test: atest
CtsWindowManagerDeviceTestCases: LayoutTests#testAddingImmersiveWindow
Bug: 167318479
Change-Id: Icabd05de71c5f4a75c7c7b1d47332924958080f7
1 file changed