Don't use main handler in ImageWallpaperTest

ImageWallpaperTest was creating an Engine that was run on the main
thread. Instead, pass a mocked handler to the Engine so that the main
thread isn't unnecessarily doing work in a test which may cause other
tests to fail.

Test: atest SystemUITests
Bug: 149204632
Change-Id: Ifc5f52bc33cd661c2b7818ccf7e8dafedd60d6c9