Make magnifier show up opposite touch

Touch on left -> magnifier on right, touch on right -> magnifier on
left.

No dynamic switching for now, just affecting initial placement.

Bug: 179499370
Test: Bring up crop UI, drag handles on the left and right, observe
      magnifier doesn't show up beneath thumb.
Change-Id: Ib020b99fec41637bce4e1797412afbdbd9447ab2