Pre-revamped UI cleanup grid file rename & move (1/2)

As part of the revamped UI flag removal process, rename files to replace
original. Also move files placed under model folder into picker folder
to be more uniform with existing revamped UI file organization.

Bug: 262780002
Test: manually verified functionalities still work throughout the
wallpaper picker app

Change-Id: I145edf632f8716169ba14fd0c83314f4dc305c5b