Adapt ImageWallpaper to support lockscreen

Up to this point, ImageWallpaper assumed ImageWallpaper assumed that it
was running either on the home screen, or on both screens. To make it
work on lock screen only, a few adaptations are necessary.

Bug: 253507223
Test: atest CtsWallpaperTestCases
Test: atest ImageWallpaperTest
Change-Id: I2b9a4b03cae88f41e9d3edd139ebf8d983af4881