Add a11y setting for disabling touchpad system gestures

Screenshots:
* The new setting: http://shortn/_9JL6nMS3cR
* The disabled "Learn touchpad gestures" button and accompanying footer
  when the setting is off: http://shortn/_FTcNTQAGYj

Test: disable the setting, check three- and four-finger swipes on the
      touchpad stop working; re-enable, check they work again
Test: check the toggle and heading both hide correctly when flag is off
      or no touchpad is connected
Test: check the "Learn touchpad gestures" button is disabled with an
      explanation when gestures are disabled
Bug: 353947750
Bug: 374965372
Flag: com.android.hardware.input.touchpad_system_gesture_disable
Change-Id: Ie7a6ea4e9ddd34710d07f78ab96598207aac4228