Configure existing ThemePicker Robolectric tests to run with atest. am: 348b8b742b

Original change: https://android-review.googlesource.com/c/platform/packages/apps/ThemePicker/+/2328233

Change-Id: I0930a719747bef3d8085fd9f4f132c4789a4b2ea
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>