[CD] Handle trackpad back gestures on connected displays

This CL enables trackpad back gesture handling on connected displays in
EBGH. It has a number of known issues, which are addressed in subsequent
CLs:

1. Exclude regions on connected displays are ignored
2. InputMonitors are not correctly registered/unregistered, when the
   isEnabled state of EBGH changes.
3. The back arrow always shows up on the main display.
4. Pointers are always pilfered on the main display.

Bug: 382774299
Test: Manual, i.e. verified that trackpad back gestures work on
      connected displays.
Test: presubmit
Flag: com.android.window.flags.enable_multidisplay_trackpad_back_gesture
Change-Id: I86fd2f7d55aaacbcd3cb80ff6529702cc8d880df