Execute flicker tests with device without wallpaper

Currently it fails with

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.ComponentName android.app.WallpaperInfo.getComponent()' on a null object reference
    at com.android.server.wm.flicker.launch.TaskTransitionTest$Companion.getWallpaperPackage(TaskTransitionTest.kt:240)

Fixes: 225350160
Test: atest FlickerTests:TaskTransitionTest
Change-Id: I516520e806fc17e75c2cdfe07116f7b84fdd5adb