[CD] Handle trackpad back gestures on connected displays

This CL enables trackpad back gesture handling on connected displays in
EdgeBackGestureHandler.

Bug: 382774299
Test: Manual, i.e. verified that trackpad back gestures work on
      connected displays.
Test: presubmit
Test: Automated tests in follow up CL
Flag: com.android.window.flags.enable_multidisplay_trackpad_back_gesture
Change-Id: I43d3a06c6c32fc2da1a3956bfe477a711e7aa17a