Regarding the accelerometer question:
1) Theoretically, A gyroscope is sufficient to cause the rotation of the screen of our phones. A gyroscope measures angular velocity.
2) But, with just a gyroscope, we cannot determine how quickly do we need to switch the screen. We also need to measure acceleration for that. To have that flawless rotation of our screen, we need both a gyro and an accelerometer.
3) Most embedded processors have both gyro+accelerometer working together, like gone are the days when we used to use gyro alone. All of our smartphones in today times, come with both gyro+accelerometer together.
So, for us to perfectly make the screen rotate on our smart phones, we need both gyro+accelerometer. So, the answer will be "All three".
