Improve comparison of default wallpapers

This adds handling of the cases when comparing a null wallpaper
component to the default wallpaper component should be considered
equivalent. This will avoid changing wallpapers when resetting to the
default wallpaper if the default wallpaper is already configured.

Bug: 296508731
Test: atest WallpaperManagerTest
Change-Id: I723cd3f07caf3e7035ba01361f76fbc9a2183950