Add a flag to not disable back gesture in launcher

Desktop mode prototypes enable freeform tasks on top of launcher.
Launcher is still partially visible in the back which means that
launcher will disable back gesture for the freeform tasks.

When desktop mode prototype 1 or 2 is enabled, skip disabling the back
gesture on launcher.

Bug: 259280363
Test: manual, enable desktop mode and launch an app, try back gesture
  from edge

Change-Id: I1d5eb81c7b28fb452c9566e358d77a5b2c19d450