InputSettings: add touchpad tap dragging setting

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)
Test: atest com.android.server.input.InputManagerServiceTests
Change-Id: Ieb9ba8961366d1cc9c6cd98e500b5be8f3b2ecac