Add isImageWallpaperWindowVisible() util method

This method will be used in e2e tests for checking wallpaper's visbility
when display mirroring switch is enabled / disabled.

Bug: 392787946
Test: EnableDisplayMirroringSwitchTest DisableDisplayMirroringSwitchTest
Flag: EXEMPT adding utility methods
Change-Id: I6ffdb6c52ec213e05931981ecf85cbcace7f50aa