touchpad: add tap dragging configuration option

Support for tap dragging already exists in the Gestures library, so we
just have to pass the value in as a gesture property.

Bug: 321978150
Test: with the flag enabled, run `adb shell settings put system
    touchpad_tap_dragging 1`, try tap dragging (tapping then immediately
    putting your finger back on the touchpad and moving it)
Change-Id: Ib1a25899d30ae612fdd6fe0fcdad38a2669dc39b