Add an option to wm density command to set the density ratio.

With the option -r the density ratio will be set to preserve display
size on resolution change:  adb shell wm density [-d DisplayId] -r

Flag: com.android.window.flags.enable_persisting_display_size_for_connected_displays
Test: Manual test
Bug: 401521173
Change-Id: Ie2c0f61a145d87d613dd66153799561437828b7a