Does minimizing full-charge time for Li-ion batteries increase their longevity?
Or do smartphones take this into account and somehow reduce charging current or flicker charging on and off once a full charge has been reached?


They do.

Any properly designed modern device with a lithium-ion battery will contain a charge controller which is aware of these issues, and which will slow down and ultimately cut off charging when the battery is nearly full. (The same charge controller is also used to display information about battery charge state on your phone.)

The AC adapter is not part of the charging process. Its only responsibility is to provide power to the device, whether it is charging or not. Using a higher current charger will not overcharge the battery, and a lower current charger may simply fail to charge the device at all.