Improve starting offset calculation

The previous implementation did not take into account the various
expansions that happen for OVERSHOOT and LEFT and RIGHT rectangles, thus
causing the initial shape to be an oval instead of a circle.

Test: manual - triggered smart select and verified that the intial shape
is a circle under the touch point for both single line and multiline
scenarios

Change-Id: I75175bc3972db3f0608ef364109e0571268da4af