touchpad: add touchpad system gesture setting

This new setting is implemented as a simple boolean check before
handling three- and four-finger swipe gestures. The only gotcha is to
make sure we end any ongoing swipe when they're disabled, so we don't
produce an inconsistent event stream.

Test: disable the setting, check three- and four-finger swipes on the
      touchpad stop working; re-enable, check they work again
Test: $ adb shell 'stop && setprop log.tag.InputDispatcherVerifyEvents
                   DEBUG && start'
      then toggle the setting while performing gestures, check there are
      no crashes
Test: atest inputflinger_tests
Flag: com.android.hardware.input.touchpad_system_gesture_disable
Bug: 353947750
Change-Id: I4333e3d3bb6f89d29332c701937ea4c870b1cbc5