Assumptions: x is the horizontal axis, and increases when moving from left to right. y is the vertical axis, and increases from bottom to top. (touch_x, touch_y) is the point selected by the user. (center_x, center_y) is the point at the center of the screen. theta is measured counter-clockwise from the +x axis. Then:
delta_x = touch_x - center_x
delta_y = touch_y - center_y
theta_radians = atan2(delta_y, delta_x)
Edit: you mentioned in a comment that y increases from top to bottom. In that case,
delta_y = center_y - touch_y
But it would be more correct to describe this as expressing (touch_x, touch_y) in polar coordinates relative to (center_x, center_y). As ChrisF mentioned, the idea of taking an “angle between two points” is not well defined.