Support for WM.LayoutParams.PRIVATE_FLAG_IS_SCREEN_DECOR.

Allow any process with the STATUS_BAR_SERVICE permission to add a
window that would be considered a screen decoration by the system
similar to the nav bar or status bar.

Test: bit FrameworksServicesTests:com.android.server.wm.ScreenDecorWindowTests
Bug: 68002203
Bug: 64148922
Change-Id: I8c78b82c10c638b54a528a77b96f5bf8dd18bf6f