Return correct IME policy for external displays from DisplayWindowSettings.

Currently if a display sets a IME policy it's not respected and DisplayContent overrides it. Moving the logic for setting the policy to LOCAL for desktop mode to Window Settings to only override when no policy has been set.

Test: atest DisplayWindowSettingsTests
Flag: com.android.server.display.feature.flags.enable_display_content_mode_management
Bug: 434276780
Change-Id: Ibcdb4055e1129ca31cbf3d41abf88015a6c7ad71