Fix back gesture navigation in Launcher with DW

The windows back functionality with the
`enable_desktop_windowing_wallpaper_activity` flag enabled works as
expected and doesn't require changes in `#updateDisallowBack`.

- Added the `enable_desktop_windowing_wallpaper_activity` flag check in
  `Launcher#updateDisallowBack`
- The early return statement will get removed after flag is fully rolled
  out (b/333533253)

Bug: 330183377
Test: manual
Flag: com.android.window.flags.enable_desktop_windowing_wallpaper_activity
Change-Id: I2eaaa44da82da6cbdfe534793830d0f44c4668ba