Fix negative touch word offset that caused crashing when selecting text

Previously the offset between the touch event and the start / end of
the word could result in a negative value - we don't care or want
this value to be negative. The negative value also causes the returned
offset to be outside the bounds of the text which resulted in a crash.

Bug: 19705184
Change-Id: I4287df7778c246dd10654f1a1f1e57538e940730