Add hidden WM API to set external display density ratio.

Flag: com.android.window.flags.enable_persisting_display_size_for_connected_displays
Test: atest DisplayContentTests
Test: atest WindowManagerServiceTests
Bug: 400418602
NO_IFTTT=Ifttt added with this change, no updates needed.

Change-Id: Ic470dddeed0ca7fcdae17f28746f99d1f78254b8