[WPPG] Separate files of overriding for building different variant

We moved the files in ThemePicker that are NOT shared with WPPG to
another folder.

Bug: 239542256
Test: Tested wallpaperpicker2Debug, themepickerDebug and googleDebug
could build

Change-Id: Ie90fcc0d614b10928bc948c03decc0d83d47f80e