Move mouse selection handling logic to Editor.

- Remove mouse selection related logic from
ArrowKeyMovementMethod and Touch.
- Extend drag accelerator to support character based
selection by dragging and use it for mouse drag selection.
- Enable word based drag accelerator even if the clicked
position is not on a word.
- Disable y-coordinate adjustment for word based drag
accelerator when the user is using a mouse.

Bug: 19544351

Change-Id: I1b29a65be3d321ac472f8c5b70ab6fee4e895964