Add a new field in DisplayContent for saving forced density ratio

This value is used to update forced density in external displays when a
resolution change happens. When the resolution change happens initial density
changes, but the ratio between initial and forced density should not change to preserve the size of the UI elements.

Flag: com.android.window.flags.enable_persisting_density_scale_for_connected_displays
Test: atest DisplayContentTests
Bug: 392855657
Change-Id: Id30f1c2fedd939d3191c28d25a2c465792ff799b