Fix wrong pointer count logic for trackpad back events

ACTION_DOWN reports pointer count of 1, and then subsequent ACTION_POINTER_DOWN reports increasing pointer counts. ACTION_MOVE events have the correct pointer count, hence the logic to check pointer counts need to be updated

Fixes: 275471396
Test: able to trigger 3-finger back
Change-Id: Icecda6bc59842598e4390514a9491cc6df078ce1