DocumentsUI: Move layout folder to be behind flag M3 disabled

This folder was left behind by accident on previous CL ag/30392873.

Test: Manual build, deploy apk and start it on DUT with flag off
Flag: com.android.documentsui.flags.use_material3
Bug: 377771271, 380369329
Change-Id: Ibc86ea5ea194aa425dc1d6fc41e9545cd385a8ac