FocusableNode: Use getClientRects()[0] instead of getBoundingClientRect()

In iOS7 a bug(?) means that getBoundingClientRect isn't relative to the
viewport, where getClientRects is. To make sure the relative
calculation is always correct, always use getClientRects.

Bug: 67063
Change-Id: Ib935c18274e1c065af9bad6a92e30a6073d12e1e