[DocsUI ZIP] Add a DocsUI ZIP flag

The `zip_ng` flag will be used to protect the development of new
ZIP-related functionalities in DocumentsUI. In particular, we are
thinking of using it to gate the upcoming "Extract all" functionality.

Test: m DocumentsUIGoogle
Bug: 382550591
Flag: com.android.documentsui.flags.zip_ng
Change-Id: I2ddec16941e58e6b12641e0cfd096c43400e215f