Check for nullness in WallpaperService onDestroy

There are cases where the WallpaperService API is misused and an instance is created manually. In those cases, onCreate is not called as expected, but onDestroy is.
As mBackgroundThread was being created `onCreate`, it ended up being null in onDestroy.

Adding a null check to avoid breaking those places.

Bug: 275361339
Test: androidx.wear.watchface.editor.EditorSessionTest#commitWithPreviewImage
Change-Id: I89076da179493fa282f3e59895cfa4f4bfb6bdfc