Option to enable/disable sw keyboard auto pop up

I have recently updated to 2.3alpha3 and saw that a lot have been changed in past months. Unfortunately, not all of the changes made me a happy camper. For example, now keyboard is not popping up automatically when I'm reviewing my cards with type in fields. And this adds substantial lag to a slow by nature process for reviewing on a touchscreen. From [this thread](https://github.com/ankidroid/Anki-Android/pull/304), I've learned that software keyboard auto pop up was disabled due to it's significant reduction of a view port on a cellphones. I feel your pain, people, but on a tablet it's not the case for the most time, so I think we should have an option to suit the best for both worlds. So, with this commit I propose a compromise: new preference. I know, that we should reduce the number of preferences, so an average user should not be puzzled with a slew of strange options. But personally, I would prefer having this, than having to do an extra tap each time. What is your opinions on this?

@ray-pixar and @ospalh would probably have some thoughts on this.