InputSettings: add touchpad system gesture setting

Test: disable the setting, check three- and four-finger swipes on the
      touchpad stop working; re-enable, check they work again
Test: atest com.android.server.input.InputManagerServiceTests
Flag: com.android.hardware.input.touchpad_system_gesture_disable
Bug: 353947750
Change-Id: Icf05b29d733525113dc9fe5869186c6b5e0644e5