Text selection: fix some issues with drag accelerator

Couple of overlapping issues that this CL fixes. The major change is to
offset the selection vertically this is similar to how the handles work
already (i.e. touch target is below what's being selected), and still
allows the user to see the selection start / end without covering it with
their finger.

This change fixes multiple issues:
1) Previously to ensure the finger wasn't covering the selection a
   preceding or following offset was taken, this made it difficult to
   select certain words and it made text on the edge of the screen hard
   to select (b/21098345 and b/19965619)
2) The use of preceding and following on the word iterator was not correct
   allowing grapheme clusters to be split, now the offset is calculated
   with getWordStart/End which respect grapheme clusters (b/21045116)

Bug: 21098345
Bug: 21045116
Bug: 19965619
Change-Id: Id8392426cce20ad0ff47a4279c92f6ed1b0ad30e