Update preference constructors.

Delegate to the most general constructor and follow the guidelines in
  http://developer.android.com/reference/android/view/View.html#View(android.content.Context)
and:
  http://stackoverflow.com/questions/8113621/which-constructor-to-be-called-for-view
Merge pull request #21 from flerda/v2.1-constructors

Update preference constructors.