Fix initial selection when focusing in Firefox

* Move the code which fixes the selection to the first content offset
  to documentOnFocus, from ui.surface#initialize.
* Take the mouse events out of the event sequencers as 1. they don't need
  to be there and 2. the event sequencer is detached on blur, so mousedown
  wouldn't fire on focus.
* Set the initial dm selection to null so it notices a change to 1,1
* Kill dm.Surface#purgeHistory as it's never used and doesn't answer
  the question of what happens to the selection.
* Update various tests which assumed the surface initialised with a range.

Bug: 63943
Change-Id: I9f8b840ce185ef71760000d275eb9474879c077e
Merge "Fix initial selection when focusing in Firefox"