Extract desktop-first decision factors into methods

This CL extracts the desktop-first decision factors into dedicated
methods.
This is a refactor and no functional change is expected.

Flag: com.android.window.flags.enable_display_windowing_mode_switching
Bug: 397557203
Test: DesktopDisplayModeControllerTest

Change-Id: Id13447c55cc11e80cc13b6c64423115c01d58829