Move WallpaperPicker to /product in ub branch

This is the branch currently imported in the internal master
repo manifest. Because the app is moving to /product in AOSP
and the build system will enforce it's not in /system, move
it to /product in this unbundled branch as well.

Bug: 122353713
Test: built aosp_arm64 in master
Change-Id: Ice735ae9e7f94559d7e0e4bed4fd72c4c3fc1b84