Touchpad: add tap dragging setting

Screenshot: go/ag-26012379-screenshot

Bug: 321978150
Bug: 324058706
Test: with the flag disabled, check the new entry doesn't appear
Test: with the flag enabled, toggle the setting and check that it
    enables and disables tap dragging (tapping then immediately putting
    your finger back on the touchpad and moving it)
Test: with the flag enabled, check the setting appears in search results
Test: atest SettingsRoboTests:com.android.settings.inputmethod
Change-Id: Iccfbe842ff655ea295edc764cfc400709948df7d