Keep resolved windowing mode of existing tasks

This CL sets the windowing mode of the existing tasks explicitly to keep
their resolved windowing modes remain the same after the display
windowing mode switches back and forth.
Otherwise, the resolved mode may change if a task’s override windowing
mode is UNDEFINED (i.e., inherit its parent’s windowing mode).

Flag: com.android.window.flags.enable_display_windowing_mode_switching
Bug: 391255364
Test: DesktopDisplayEventHandlerTest
Change-Id: I4ae5be3874e9be603da1f6a38bc473f80dfe7364