Fix pixelated wallpaper image shown on external display

Root clause: the wallpaper image is downscaled during the bitmap decoding process because the default bitmap density is non-zero for external display.

Also, relax the max display to wallpaper image ratio back to 1.5 as we identified the root clause of the image quality problem.

Flag: android.app.enable_connected_displays_wallpaper
Test: connect to an external display to check if the wallpaper has good quality on the external display.
Bug: 403277285
Change-Id: I04672881d13c7910aaa496ea0177d08f9cdfba39